Sometimes you just need to step up the program and use a big old Hex Cap bolt. Hex head cap screws or bolts have got the heft that you are looking for in marine grade 18-8 Stainless Steel so it will be able to resist corrosion. Remember that Stainless Steel will sometimes Gall (seize) when tightened under load -- especially with larger size fasteners like Hex Caps. To prevent this apply a dab of Anti-seize compound before tightening the nut.

3/8 Imported Hex Caps will be fully threaded up to 3/8-24 x 1-1/4. Anything longer will have approximately 1 in. of thread.