I’d want a hoist on each post. Gives much more control. And I’d go with chain blocks, again for more control. I used to work in a boat yard. Got quite good at working several chain hoists at the same time. It’s not a job to rush really.
But ! If @barking mat purchased the lift for you. You would then be able to get a boat, lift and move it to somewhere that would not pose the getting wet issue.
Could you (or a professional firm) construct a temporary scaffold? Use that to get it on to blocks. Bring them back when you need it back on the trailer.
We used to have this at the club, it was mobile and used turfers for lifting but with the cost annual testing we ended up scrapping it as it didn't get enough use. Not good pictures but my boat only just fitted in and the top did deflect in a little. I used it to lift my boat from the launching trolley to drop the lift keel into a hole I dug below.
Atleast a dozen times ayear. That is the plan lifting gear on each corner. And the on ei sued in a yard a few times uses block n tackle and as you say its better control. Works a terat. I am just going with each post having top supports but front ( atleast ) and back removable.
