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1) Experienced player are not phased by the Downed state. Even underwater, it really should be a simple matter to swim to them and finish them off. I do think that your movement speed should be drastically reduced, because right now you move fast enough that it's possible to avoid the Finish just by moving.
2) This is absolutely vital for people to learn: YOU CAN SET YOUR UNDERWATER HEAL/UTILITIES/ELITES INDEPENDENTLY OF YOUR LAND SET! Please, please do this! You'll swap between loadouts automatically when you go between land and sea. As a corrolary, there is a lake in the Mists. Practice with your underwater set so you know wtf your skills do.
3) The underwater cap point and the sharks aren't the only thing this map has going for it. There's also a pirate cannon on the platform overlooking the ship, but you need to load it with cannonballs that you collect from the ship (one cannonball = 5 shots, of course...). It's not quite as powerful as the trebuchet, but you can aim it much more easily. It hits almost all of the land areas in the map. Also you can find beer bottles on the top deck of the ship that grant you a few seconds of invulnerability.
Haven't gotten to try the underwater combat yet. But, I have to say that I don't feel the downed state adds anything of value to the game. Personally, it annoyed me greatly and I would rather just be dead and respawn.
The author's frustration seems to stem from his overwhelming internal need to "Kill" players. I'm not saying whether this is good or bad, not my point. What I'm saying is that it is not 'the' point of sPvP. The point is to win the match. The match is won by controlling points. This is far and away more important than killing other players. If you down a player that was trying to contend for the underwater checkpoint, and the result is him tediously swimming away to safety, then well done! Mission accomplished, now stay at the point and secure it. Don't swim after him frustrated that you didn't get your ego stroke point. You've already done what your team needed for the team to be successful. A reprioritization of the player's personal objectives is all that is needed to embrace and thoroughly enjoy team warfare. It's not about the individual statistics, it's about the team victory.