You can try this:
Make a hoe, get rocks and build up the land underwater until it is reachable when the tide recedes. I have done full dock foundations this way from a raft and my friend and I build a land bridge over the ocean for hauling some ore with a cart. It is also the secret technique to retrieve items from being stuck in tar pits in plains

I just build a tower over the location then jump into the water from the tower.
But considering it's in the plains, it might just be easier to go get more iron.

you can use a hoe to raise the ground and collect them that way

Originally posted by dr.toast:

If single player, you can always pop the devcommand and enable fly.
That lets you dive right to the ocean floor at will.
Personally I'm okay with it cos its a bit BS that stuff sinks like that.

Also for legit ways, building a small ladder and "diving off" it spamming e to pickup can work depending on how deep.

That's what I do, if I cannot get to things by jumping off the bow of a boat.
Note: to go to the bottom of the water, use the control key.

Originally posted by dr.toast:

If single player, you can always pop the devcommand and enable fly.
That lets you dive right to the ocean floor at will.
Personally I'm okay with it cos its a bit BS that stuff sinks like that.

Also for legit ways, building a small ladder and "diving off" it spamming e to pickup can work depending on how deep.

The image in my head of a viking suddenly flying up and then into the ocean seamless causes me the chuckles. I guess it is a form of the after life so it could make sense.

theres also a mod for making items dropped in water float.

If single player, you can always pop the devcommand and enable fly.
That lets you dive right to the ocean floor at will.
Personally I'm okay with it cos its a bit BS that stuff sinks like that.

Also for legit ways, building a small ladder and "diving off" it spamming e to pickup can work depending on how deep.

This is one of the frustrating things about Valheim. You can't dive or swim.

So the only way is to jump off a high place, and aim into where your iron nails are. Then frantically click your use (pick up) button. I think 10 ft might be too deep however.

Your only hope is to somehow jump into the water from a height. Your momentum will carry you briefly underwater and you can grab the nails if you're fast enough.

if it's that close to shore try lvl'ing and raising the ground out to them.

I saw a video about how you can build roofs descending into the water. If you run under them, you can force your way under water. Might be easier to just mine more iron, though. I don't recommend taking ships too close to the swamp, for a number of reasons. Find another biome close by to park it.

I feel ya bro as I lost 30 iron ore to the sea once. Once.

If the area is near shore, try terraforming the land up. Do be careful that it doesn't roll back down hill, maybe wrap around the area first.

Last I played, it was possible to generate more land by walking to a high ledge and hitting it with a pick. The area around the pick gets shifted to a new height, pulling down high land a little bit and pulling up the low land a LOT. It saves a ton of stone, just start high and slowly flatten the land out and to the side.