I've lived aboard for over 10 years. While I have seen a few marinas that allowed for rental of small storage spots it has been very few.
I bought a 15' pontoon boat with a canvas cover and for a few bucks I have an on the water storage of 15'x 8' 3' high.Enough room for 2 kayaks, a windsurfer, 4 outboard motors, high pressure sprayer etc. Others on my dock have offered to "join in" on purchasing an old, wood cabin cruiser and making it into our workshop with bench and vice etc.
Prior to the pontoon boat I made flyers and put them on the garages of a near apartment complex. Many apartment dwellers don't have cars and I ended up with a 1 car garage for $25 a month.