Deep water where? Middle of the ocean or closer to the shore?
Unlike other mechanics normally ruled by physics, free fall is implemented pretty convincingly in Valheim. If you're willing to put in the effort, you could build a platform tall enough to let you get stuff from pretty deep waters. The higher you climb, the lower you dip, and there is no fall damage if you land in water.

Other than that, the community will likely be able to tell you what mod you can use to get stuff back.
Or use the -console parameter to start the game and get familiar with 'devcommands' -- you can cheat pretty much anything back into the game.