Gonna hafta go with shallow water, subnets, mines, and tiny islands you don't even see...
Then of course...there's the blind lookouts not informing me of land based guns (also in shallow water)...
I
really hate the Marshalls and now refuse to patrol there...everytime I go, the weather is bad...or the waves are high...and I end up dying mysteriously while patrolling with TC up (no higher then 1024, and usually closer to 512). One time I know it was shore guns as it pulled me out of TC after the initial hit (which doomed me...). It's rather nice how the lookouts are blind...
I think the last one was due to a subnet, but I can't be certain...