How to recover dwarf corpses lying in a cavern lake?

Dwarves will not willingly path through water that's at least 4/7 deep, so as long as the lake remains a lake, your fortress' belongings will remain at the bottom of it.

The easiest way to recover the corpses and gear is then to drain the lake and simply walk through the mud to reclaim your items. One way to do this would be to dig a drain leading into a lower cavern layer, making sure to leave one tile of solid rock between the drain and the lake before you need it. Once the drain is complete, you can channel (d -> h) the tile keeping the water back and let it safely inundate the lower level.

Moving water can drag along objects and creatures, including your dwarves, so it might be a good idea to install a floor grate or horizontal bars right above the drop(s) in your drain, to keep what you're trying to recover from washing down with the water.

If you're mostly interested in not having your dead dwarves come back to haunt your fortress, you can engrave and place a memorial slab, which will put your dwarves to rest just as a grave would.