I spoke briefly with a friend who has a boat, that he was considering bringing here. He said that due to the coastline and tides, there aren't many places that have waterfront dockage. You can find it in the Golfo Dulce (between Golfito - Zancudo) and in Puntarenas there are sometimes homes available with docks on the estuary.
Right now there are 2 small marinas operating in the Golfo Dulce, and there is Los Sueños. The Flamingo Marina should re-open at some point under new management. There are several marinas going through the approval process, but that of course means no fixed date for them to even begin construction.
For your residency you need to find a good lawyer who specializes in this, but I believe that your investment would be tourism, so that would help you obtain resident investor status at the lower level for priority sectors.
Here are some links for the marinas:
Banana Bay Marina - Golfito
King & Bartlett - Golfito
Los Sueños