Stuck rudder hatch cover

I am replacing the sealant on my Knysna ski as it was letting in water to the rudder hatch compartment.  I cant for the life of me untwist the hatch cover.  I have tried:
*sitting the hatch cover in warm water for a few minutes
*Applying a silicone spray
*Using pliers to try get any kind of movement.

Husband cant do it either and we are fearful of breaking it.

Any advice?
If i do get it undone, i will apply some silicone grease to the threads

thanks

I leave mine off, no need for it, that way you can routinely check the connections in there. Water gets in there up the rudder tube anyway. If you had a rudder failure and wanted to rig up an emergency connection on the water you dont want to be fiddling with a sticky hatch

Can you take the screws off the flange rim and remove the whole assembly to make it easier to disassemble?

(Im not familiar with Krysna skis, but thats the normal arrangement if your boat is different altogether, then ignore this)

Try chilling the cover rather than warming it.  I’d suggest resting a bag of ice on the cover (without contacting the surrounding deck surface).

Yes I took the whole assembly out so I could put fresh sealant down.
Am thinking of maybe screwing the assembly into a plank of wood and then trying with the pliers again in order to get better leverage 

Well just a note, if you are trying to make the hatch watertight you are wasting your time! You have a great big hole where the rudder shaft comes out the hull and another two where the cables come out the footwell.
Fresh sealant, not sure what you are talking about? Why do you need to replace sealant that should NOT be there in the first place.
As for unscrewing the hatch I usually get a block of wood that fits into one of the hatch tangs and hammer it, that's if my normal tool does not work. Just a piece of wood with two screws screwed in at the appropriate pitch and use it as a spanner, the screws face down and fit into the hatch. Normally works a treat.

I had the rudder hatch on my first gen V10 get stuck a few months ago. I used 2 hammers simultaneously to carefully hit/twist the cap. I assume it was stuck from salt dried up in the threads, so it just needed a bit of jarring to loosen it up.