I get that the whole idea of oceans is to separate players that live on different continents but it's VERY punishing for anyone who lives on an island, and I think island bases are quite neat.
Yes, if you have a big island you probably can progress your industry along for quite a while but at some point you will start running into no more places to mine issues and would force you into making a mining outpost (where you either move all your smelters and so on, or are forced to transport hundreds of ore stockpiles back to your base, which is not that bad considering you could just go with a knarr, fill it up, and then port back but it just seems like a chore).
How about allowing to mine under the high seas once you reach -8 or -9, so it's not something easily accessible from the start, but once you get into the stage where there is no more deeper levels, you could expand your mines under the ocean as to not run out of space.
If you really wanted to make it a pain in the ass you could make it so the supports break more easily under the ocean/just more cave-in tiles/naturally tougher rocks/etc. but tbh i don't sea a reason for it to be any harder than just normal mining.