depression, as i'm finding out, is a very tough thing to deal with. very, very tough.
you're definitely making a step in the right direction by posting about it here. even though we're just some guys (and a few girls!) online we can definitely offer advice.
having a therapist would presumably help, i can't say for sure though.
the only solid advice i can give you is to try and pick up another sport, or even hobby, that would be less exhaustive than water polo. try tennis... get on a court with a buddy and just hit some balls back and forth. practically endless enjoyment.
oh, and the last thing you should be worrying about is the 'backlash' you'd theoretically get from your coach/team. I quit marching band this year due to not wanting the stress of 20+ hours a week of practice for 4 months, few other reasons... i went to my band director and flat out told him, i don't want to do marching band this year. i gave him my reasons and was very level with him.
i'm missed in marching band, but when my friends/peers ask me why i'm not doing it, i give them an honest answer- i just plain don't want to anymore. and none of them give me any lip for it. the best thing you can do is be honest about it, and if your peers/coach happen to dislike you for your own decision, just avoid them. they'll get over it, without a doubt.