That lifting davit looks to be home made. Not that it's a bad thing but there is no way to determine its capacity. Judging by the fact that there is no bearing for rotation I'm thinking probably less than 500 lbs.
Turn it towards the dock or deck and rig up a board to sit on. Try your weight and judge twist and strain. Then add a couple kids or weights or the Admiral (might be fun). Keep close to the ground so you don't have far to fall. Have someone checking the mounting plate, if it starts to show strain or flex better abandon the test.
pete