.* I would also look at a dock door for each unit, or a common dock area.* Do you have plans to go ahead and finish office and restroom out for each unit, or are you going to leave it blank and build   the inside to suit? 

Welcome @Greg Gaskill Great to see another awesome investor come aboard the ship of success.  

Well  short answer is they don't.. or they have capital to buy them.. or they do what @Shiloh Lundahl does and puts together investments for others so you have in essence a small syndicate or partnership in the owner ship of the assets.the other way folks do it is the BRRR which some people seem to think is new concept but it was the way investment property was bought pre 08 most of it any way.you pay cash then refi out.. 

Hi,Lately I have been seeing a lot of boat slips & dry dock storage for sale.  

Does anyone have experience purchasing boat docks from marinas then rent the dock out?

This comes out to $4.2k that I get to ship over to my tenant for trying to move back into my own property.

No boat docks included.  

Lakefront, dock, bar within walking distance. 2 bedroom 2 back with an upstairs bunk room.

If you aren't running a tight ship tenants can ruin those on paper numbers very quickly. 

Beyond all the normal stuff you hopefully will have a dock to maintain, possibly permits for dock, water toys suchs as kayaks.