Here is a short list of some affordable rtr boats.

SW26, nice surface drive boat with upgrade possibilities.
Aquacraft Hammer EP, nice subsurface drive boat with more limited upgrade possibilities.
Traxxas Villan EX, nice, pretty fast boat out of the box with deep vee hull thats good for rough and smooth water, but expensive to upgrade.
The regular Villan can be had for less and I believe is just as fun.
Traxxas Blast, Decent smaller boat with upgrade possibilities, but I would prefer the SW26 over this for better looks and the hull has more room for upgrades.