I don't know how often they inspect them, but I do know the work's done by specialist underwater divers.
I met a deep sea diving engineer once - weirdly enough on a slow train ride through the Andes.
He explained about his job - they inspect the foundations for oil rigs, bridges, anything that's in the water really.
There are special suits for diving, of course. Special air mixtures for super deep depth, and of course heated suits for cold water dives. He's done North Sea oil rigs, and the foundations of the Firth of Forth road and rail bridges. For all of those they go down with fixed lines feeding them air (or their special deep water mixture) and of course warm water circulating the suits - it's pitch black and freezing cold.
There are lengthy stints in decompression chambers, and of course lots of practical jokes. He told us a couple of them - turning off the warm water is a favourite, and the funniest he said was giving someone a hot chocolate laced with laxatives just before he went down for a dive inspection knowing the guy'd have a 5 hour decompress before he could visit a loo or get rid of anything from the chamber....nice!
There are also several methods of engineering metal underwater - special welding equipment and techniques and so on - a very specialised field. Quite dangerous, but not more so than working in other extreme engineering environments. He works a lot less than most people and earns a lot more money......not a bad idea.......