if you dont want to use hydraulic the genie lift reference is just as good note the head can be used both ways below and above but can be used on the hydraulic version above by design of coarse
Surely the easiest way is just dig a hole. Fill it with water. Back the trailer in when you want to take the boat off to use the trailer for something else.
A friend of mine Ray Loveland (sadly died a few years back) was the best boat transport guy i knew. He had a small lifting gantry for lifting yachts up to about 27ft long or about 4 tons. It consisted of two folding A frames that he put one each side opened out fore & aft, each leg was 80mm square box, a removable cross bar went across the top think it was 60 x 30 box, two 4"x2" wooden braces one stuck in each side with wedges to stop it folding over sideways. It was set up at the balance point & a lever chain hoist each side coupled to the two lifting strops. I doubt it was ever lolered or puwered but he lifted hundreds of boats with that rig & never dropped one either. Think i have a photo of it lifting my old yacht somewhere will try & dig it out.
They've got a few of these (!) down at Port Edgar harbour, where we live. Very impressive looking bits of kit! They always winch the straps up when not in use, to stop kids (and adults!) swinging on them