Make yourself a little tool to open them when the going gets tough. Temperature expansion etc. can really lock up those screw hatches - sand too. Most people use the pop-in types for that reason. On the boat that I have the simple pop-in Becksons, I screwed in a little finger loop of strapping to pull them open.

If you go to that Viking site - you have to go back to http://vikingmfg.com and to (contact) to order.
The site is a bit primitive.