Check your Flywheel and the seat of the screw in it.
and ignition coil to have good ground
Check your spark and the flash
Check your carb and all your jets clean it out
Check the petcock
Is there a mass dirt in the tank
When this is all ok
Than take a look behind the head
I  think the top end is good when you have compression.
Is bad when entering water in it is the same f..... starting procedur
I wish you a lot ..... Frank