There is a mod that makes all loot float,

Decided to install that after I found out that apparently the Serpent Scales are coded to sink even tho the Serpent Meat is coded to float. Which meant many hours of fruitless scale hunting was apparently not because they weren't spawning but because the devs decided to make them sink for the lulz.

There are a metric ton of mods out there that really improve the QoL of the game and some that truly enhance it.

The specific mod in direct question
https://www.nexusmods.com/valheim/mods/241

That said I would love it if the player could swim underwater but I'm not holding my breath~