Blumags' portfolio of underwater magnets have a holding force ranging from 100 kg to 800 kg (220 lbs to 1750 lbs). Despite their massive holding force, they are all easily operated by hand. The handle is turned vertically in order to release the magnet from the steel surface (and should be kept vertically when applying it as well).
One underwater magnet can be used for a thousand applications, many of which may require a different interface than the standard eyenut. Thankfully, the eyenut assembly can be easily removed in order to firmly attach another piece of equipment instead. Should you need more than one bolt, the Willy series has two additional holes.