Steam is generally not dangerous (magma mist is something completely different).
I've used various methods to salvage candy in the sea. The one I'm currently using is to first determine the maximum footprint of the rectangular area needed to salvage all the safe parts of it, taking into account that each level down will require the level above to extend one tile further out. The last rock level above the sea is then dug out to that shape, and a water source is connected (usually the aquifer), with a drawbridge to regulate the influx of water.
I then flood the area to a depth of 2-3 (using a door at the exit to keep the water in, if needed) and close off the water. I then have my miner channel safe parts, i.e. along candy and rock, so when the water flows down it generates obsidian that fuses sideways. If you screw up and channel such that a tile is not supported you get a cave-in with the obsidian falling. This can be lethal, and it can be "safe". This method causes me to gradually expand an outline of the rock until the whole area is obsidianized. At times more water needs to be added. For areas I can't reach (such as magma under unsafe candy), I "build" out from the areas I can reach, and then repeatedly channel down the obsidianized rock to gradually whittle away at the magma in the pocket.
Also be aware of the dangers of magma flows. They look just like magma, but any water flowing down will vanish together with the magma. A magma flow pocket with no magma left will fill up with water, however. There is a rather cumbersome technique to obsidianize magma flow as well, which I think is described on the wiki. Beware of the danger of diagonal down-up access through (up)/down staircases, though!
In the past I've built large retracting drawbridge contraptions to drop the water after I've completely channeled away all rock on a level to completely expose the magma, but I think the wet digging method is safer and easier. Since the water flowing down immediately obsidianizes the magma it's virtually impossible for magma critters to get up (but I still won't do any mining while the more dangerous kinds are about).