The boat slip proposal is mostly equivalent to renting merely land, not any building.
For those new to the boating world, a slip is just a space to park your boat on a dock. You would then need to, k'know, have a boat to live on.
More importantly depending where your boat is, you might have to deal with extreme heat, cold, wind, flooding etc, your water and waste disposal isn't plumbed, you might be generating your own electricity etc etc. The flip side is that you get the fun of going boating, but it's a lifestyle, not a cheap flat.
Sent from my narrowboat. (Shameless plug: for sale!)
I'd be overjoyed to rent 450 sqft of land with a 5 minute walk to downtown NYC or Seattle. Especially if it came with water, 30/50A electricity, Internet access, restrooms, showers, and laundry. Day-use dockage, grills, picnic tables, and a view of the water are not as essential but are a big bonus!
It's really not that expensive to float a boat in a slip, especially if you're not burning enormous quantities of fuel to transit the coastline. You're not going to get a brand-new yacht for the price of a mobile home, but it's not $2,000 per month, and you're likely to get a significant fraction of your investment back when you sell it.
I feel like you described an RV park.
They're located in rural areas, where owning 450 square feet of land costs on the order of $50 ($5000 per 43560 sqft acre).