[Wheeler_Dealer]

I had a gang war where I wanted to go to a big rock north of Algonquin. I wanted to snipe rival gang members on the docks but didn't want to swim just in case I triggered a the police. It worked great. I sniped all but 5, then went down to the salvage yard just to the west and sniped two more so I only had to take on three up close.

After about 45 minutes searching, with help from the IGN interactive map, I gave up and cheated for a boat. Are there any boats available to Johnny?

[lol232]

Yes, they are.
You can find some at North Alderney near the Helipad, also you can steal Police Boats (Police Predators),
not to mention you can steal the ones that Peds are driving, just takes a little luck to get on them, then steal them.
I don't know any more boat locations, not even in Vanilla GTA IV.

[StonersDarkness]

there are some at the edge of Castle Gardens... pretty useful when people are on Happiness Island and you don't wanna take the heli from the Booth Tunnel.