Having to be rescued is a big fear of mine especially if it is something silly like a flat battery or running out of fuel - oh the shame.....
The other fear is being taken by a sea serpent - I would hate to be swallowed by a sea serpent and end up being his/her lunch/dinner. Imagine the pain as you are slowly digested in its long, stinking,cavernous, twirlng coils of fishy horror - I also have a small fear of being taken by a giant squid but I am not convinced they would come up from the deep dark depths to ingest me
Giant crabs are also a worry but you do not normally get these at sea - more likely - to find them whilst on leave at port.....cheers...........Geeb