The boat is the cheap part as that price is fixed. The slip is a monthly that generally has to be renewed each year. Around here the slips are ~$700/month which is about $8k a year for a tiny boat. Yes, that is cheap compared to an apartment but that does not include the hassle of travel between the boat to car to work or life which adds yet another level of time-cost. This also means you're living the SINK/DINK life without a pet because docking a boat does not make it easy to have family life unless the ship is large enough for multiple rooms.

Living in a van is cheaper but there is always a cost and trade off.