#1 I would not mount new seat bases with out a backing plate if you can get one in there.
#2 looks like that's your drivers seat. Awful lot of stress when running motor and rocking back and forth hitting waves, bolts look to big and holes drilled to close to edge to take that much flex plus those bolts should have at the least a large body washer between the head and the plate. Those sharp corners of the head will bite into the aluminum not and puts a lot of stress in just a tiny spot and over time weaken that spot. Causing it to do just what it did. Weld up those spots, and install a small backer at each hole.
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Charlie Weaver USN/ENC 1965-1979