I hereby shamelessly rip this off of Halo Reach. The theory behind it is not really complicated though; it is simply a series of assault/defense based objectives coming one after the other, though usually with slightly different spawning rules than usual. A game of Invasion consists of two rounds, and each round consists of up to three sub-rounds, or phases.
In the first phase, weapons/loadouts are quite limited, and the match takes place on a fairly small area. Objectives are simple, such as capturing a certain point or bombing an objective, and there are usually two objectives that can be attacked. If either one is achieved before the allotted time runs out, the round moves into phase two. In phase two, the equipment options expand somewhat, and the playing field becomes fairly large, close to or the same size as it is in the final phase. The objectives for phase two are also fairly simple, but failure to complete them in the re-adjusted time limit will end the round.
In the last phase, the battlefield is at its largest. Equipment options are in their full incarnations, giving the best tiers to both teams, and the game becomes full-scale. Usually, the objective for this phase is the most difficult, and CTF (or variations of it) is common. Completing this phase means the attacking team wins all three possible points for the round. (Whereas completing phase 2 but failing phase 3 means the team receives 2 points for the round, and 1 point for achieving to phase 2 but failing before three. Obviously, no points are given if no phases are completed.)
In the second round, the attackers and defenders swap places, roles, equipment, etc. and the fun begins all over again.
Spawning. The defending team may only spawn at locations close to the objective in phase one (not that they'd want to spawn anywhere else), while the attacking team has their own spawn location for phase one. The attacking team's spawn for this phase is also known as the backfield spawn point, and they may respawn here anytime for the rest of the round to stock up on war material. In phase two, the defenders still spawn near the objective; they may not use their old spawns for phase one. The attackers may spawn at points close to the phase one objective they just completed, or they may spawn at the backfield. In phase three, the defenders are pushed back to their final spawns (which sometimes happen to be the same as those in phase 2), while the attackers spawn point may advance depending on how the game's objectives are setup. The phase two and three objectives are often placed closely together and if that's the case, the spawn points usually do not change at all between phases two and three. Usually, players may also spawn by a player, but this is controlled by certain rules.
Battlefield Bad Company 2 has a game similar to this, and one may argue that Reach took the idea from BFBC2. However, I (unfortunately, and shamefully) do not have this game and have only played the demo a few times before it closed, so I don't quite know the main differences between the two games regarding this gametype.
From FPS to WW: This game needs some adjustments to be playable for a water war. All objectives must be suitable for water wars, and at least two players (one on each team) need to be running a timer to know when a phase ends. Unfortunately, seamless transitions between phases (like in the actual game) may not be possible or easy to do, so they may be omitted. Spawning, for the most part, remains similar to the spawning in Halo Reach, but I would opt to remove the restriction on fireteams placed in Reach. (The rule is that each team is divided into three fireteams, and each fireteam is restricted to certain spawn points unique to each fireteam throughout the game, except for the backfield.) I would also handle spawning on players differently.
In fact, I will go over, in good detail, my own way of setting up Invasion.
Phase one: Players are restricted to "light water weapons" only. All CPS's, hoses, etc. are banned from this phase. Players may refill via water bottles/containers only and may not use hoses/faucets. I will have to build up a good definition of "light WW" in order to carry this out effectively, or I get people picking up CPS 12k's like last time. Spawn points are fixed, with one backfield location for the assaulting team and one temporary location for the defending team. The attacking team may use a mobile spawn points, where they may set anywhere on the map they wish. However, once it is set, it may not be moved. Players also may not spawn on the mobile point until it has been set. It may be destroyed by the defenders by simply soaking it, and may not be used again for the phase after being destroyed. All rules on spawn intervals are in place for all teams and spawn points. The rule is every fifteen seconds. (i.e. a watch is positioned at all spawn points and when players reach the spawn point, they must wait until the seconds counter reaches a multiple of fifteen in order to spawn.) The objective for this round will be a conquest object; players must completely blast one out of two specified targets in order to complete this phase. The objective will be located at an area slightly far away from my house, and the time limit is three minutes but may be adjusted.
Phase two: All water weaponry is now permitted, except for hoses, WBL's, cannons, and other super-weapons. However the defenders will have access to one relatively short hose. The objective will be another conquest object, but it will be slightly more difficult. The objective is a .5L water bottle that will be fixed in place (via duct tape or something), and the attacking team must completely fill up the bottle. There will be two of these bottles, and filling up either one will win the round. (With a time limit of three minutes.) I don't want to turn it into a Soakn' Destroy round because that's too many rules to shove at players. The attacking team may now use up to three spawn points, but may only have one out at a time. They may decide to manually remove a point in order to move it somewhere else, or the defending team may destroy it. In either case, moving a point or bringing up a new one will count against their number of spawn points that they may use.
Phase three: Objective is CTF with a 5 minute time limit. The flag must be taken back to the attacker's backfield spawn in order to score. Attackers may use up to ten mobile spawn points and may have up to three of them out at a time. Both teams will be permitted to use any water weaponry available, except that only defenders may use WBL's. (I don't want water balloons launched TOWARDS my house.) Two hoses also belong to the defenders, who will guard their flag viciously. (They may however, be "stolen" and used by the attackers.) The defenders have two WBL's with which they may only fire one shot out of for each one for the round. (This rule will change in the future to allow as many shots as the team can handle, if WBL usage goes smoothly in the games.)
Of course, this is roughly specified right now. I have quite a lot of details to work out in order to have a fully playable gametype up. This game is very intricate by nature and is very likely to fail.