Yes apartments are expensive. Renting a bunk on a boat is not common. If it were available I would assume it would be cheaper than an apartment. The best way to find a boat to sleep on would be to hit the bars near where boaters hang out.
Without knowing where you may get employed, it's very hard to suggest where you should look for housing before arrival. Your best bet would probably meeting someone to share an apartment with. That seems to be a fairly common practice.