Cleats in 20' SeaCraft
 

The stern cleats are loose and there is no way to access them underneath that I can see. They are located above the vents! Do they have nuts or are they simply lag bolts and don't need accessing underneath?

Not sure which year/model 20' you have but I am guessing its an SF? If it hasn't been modified then the cleats are probably backed with a block that has been glassed over then bolted through with a nut and washer. I would imagine that if there is no other logical way to access the hardware under the cap that you could remove the vent and get access there? I am newer SeaCraft owner with a hawse pipe setup so my access to the stern cleat is different.
If you can, post a picture or two.

Cleats
 

I should have given more information! Sorry! It is a 1974 20' SF! I have the vents currently out because I just had the hull painted and the bottom of the cleats definitely cannot be accessed from there! Any help would be appreciated!

You should be able to reach them from the stern compartments - raise the lid and reach up from there. Tough but doable.

Cleats
 

I tried to access the bottom of the cleats through the rear hatch and no luck! The cleats are sealed! When I tried to tighten the screws on the cleats, they just continue to turn with no resistence! I wish I knew what was on the other end of that lag or bolt or knew how to replace them?