I'll chip in here with my realistic annual costs for my R-27-OB that is moored at the Port Of Edmonds in salt water and in a covered 30-foot slip.
1) Covered Moorage (split between my son and I): $3,120 + $3,120 (includes all taxes, elect use, environ fee, etc). Marina offers a free parking permit and free coffee and as much as you can drink....
2) Yearly scheduled September Engine maintenance for my F300 and 9.9HP kicker which includes hauling boat out/back and being towed to Jacobsen's Marine shop some 1000 feet away from marina: $650
3) Insurance with American Modern: $650
4) Diver service for inspecting underwater zincs and hull condition and general cleaning: $450 (photos always taken and included in the Emailed invoice, and this cost can vary from $250 to $450)
5) Fuel cost: Varies quite a bit each year, but I estimate it is between $1500 and $6000 for gasoline at $4/gallon. Many times my passengers will chip in to help with fuel costs.... Niceee and welcomed.

I tend to move/cruise at 30 mph if I can so this runs the boat at around 15 gallons per hour.
6) Bottom paint every 3 to 4 years: $2000. Say $500/year.
7) Misc: $100/month (estimated)
8) WA annual registration: $500 which deceases each year.
Thus
for me (with son's help with moorage cost) it's some $7,000 per year with light boat use and $1500 in fuel cost, and with constant use and $6000 in fuel cost it comes to $11,320 per year ... or on low end $583/month and on high end $943/month. This cost is reduced depending on who's using the boat (see below as family members use the boat at times).
As an aside, my son was insisting I keep the boat in the water and nearby as he kept saying if on trailer the boat simply will not get used frequently. I was humming and harring for a while and in the end son said he would pay 1/2 the moorage cost.... that convinced me and I stopped humming.

Also, I have two of my three grown children living within 2 miles of me in Edmonds. My son and my eldest daughter know how to operate the boat and have my permission to use the boat at any time they wish. When they do they pay for its use (fuel and marina docking costs etc). The agreement is when boat returned they top up the fuel tank and fresh water tank and pump out. Thus the boat is always ready to go. The 5 grandkids love to go out on the boat and we crab and fish when we can. The grandkid's experience is an invaluable/Intangible cost along with the great times we have as a family all together on the boat. My third grown daughter lives some 900 miles away and when she visits we always take the boat out for cruising and one of her favorite spots are the San Juan Islands and Gulf Islands up in Canadian waters. In addition, she loves to sleep on the boat (condo on the water she calls it) for several nights when visiting; she loves the fact it's 'quiet time' for herself.
Sometimes we ferry people across from Edmonds to Kingston, Clinton, Poulsbo, Blake Island, Tacoma areas and at times to Port Townsend areas, Friday Harbor and Roche Harbor, and will at times offer to return them back to Edmonds.
All of these extra experiences carry an intangible and invaluable cost savings IMO, and serve as life-time experiences/memories.