I had problems with boats like that before, but I haven't lately. I quit leaving a boat in my harbor and just craft it when its time to go sailing. I even tried leaving the boat in deepish water and yeah... still happened and I lost parts to the bottom of the ocean.
Then one day it just stopped happening and not sure why. I believe it is going to be one of those mystery issues until the game is completely done lol

I disable boat damage with Valheim Plus mod.

It's a synching issue, the boat's position synchs between you and your friend and due to the small lag it drops a bit into the ocean effectively taking fall damage. You will need to either turn off damage or increase boat HP. Dedicated servers have the same problem.

Perhaps you need to learn how to steer. Build a longboat where there aren't any rocks and practice. Boats have intertia, so you must anticipate.

Originally posted by Polonius Ulf:

Perhaps you need to learn how to steer. Build a longboat where there aren't any rocks and practice. Boats have intertia, so you must anticipate.

This happens in deep water with no rocks or enemies. The boats constantly take 10 damage every 30 seconds.

Build a workbench near your boat and use your hammer on it to repair it. Always carry at least 10 wood with you when sailing so you you can hop out at any patch of dry land for more repairs. It's really weird that you could take enough damage to 'not make it to the next island' ... unless mods something something.

https://valheimbugs.featureupvote.com/suggestions/158680/boat-taking-damage-at-frequent-intervals

I found this and it is very similar to what happens to me but I have no idea how to solve it.

Originally posted by Alji:

Build a workbench near your boat and use your hammer on it to repair it. Always carry at least 10 wood with you when sailing so you you can hop out at any patch of dry land for more repairs. It's really weird that you could take enough damage to 'not make it to the next island' ... unless mods something something.

I have no mods installed. I play just me and a friend. But it happens to us regardless of who drives the boat or if there is only one in the boat sailing.

Initially when we sailed for the first time it did not happen. It started to happen after we eliminated a troll from the boat with arrows on the edge of the black forest. And since then it happens constantly, it is so often that it even destroys the boat before reaching a new island. We have tried building a new boat (Karve) 3 times and a raft 2 times and it happens again.

I have only ever seen that in heavy seas, storms or sailing very fast with a strong tail wind. I take it as a warning to back off on the sails. I believe you will find if you go to half sail that you will be able to continue without damage. If that doesn't solve the problem, maybe there is some bug that I haven't experienced.

Originally posted by Alji:

I have only ever seen that that in heavy seas, storms or sailing very fast with a strong tail wind. I take it as a warning to back off on the sails. I believe you will find if you go to half sail that you will be able to continue without damage.

I know what you mean but this is something different because it happens even with clear skies and calm seas even regardless of whether we have little, half or full sail.

I have the same problem, my solution is using the devcommands just so I can repair it without a workbench so the boat can make it. It seems like it happens if you are experiencing connectivity issues between pc's. Another thing I did that reduced the frequency of this occurrence is if you run your world off of a dedicated server rather than hosting thru game directly. In this case my pc can handle running both side by side but Im sure if I had a second pc it would be much more smooth sailing.

Originally posted by Mr_Cleene:

I have the same problem, my solution is using the devcommands just so I can repair it without a workbench so the boat can make it. It seems like it happens if you are experiencing connectivity issues between pc's. Another thing I did that reduced the frequency of this occurrence is if you run your world off of a dedicated server rather than hosting thru game directly. In this case my pc can handle running both side by side but Im sure if I had a second pc it would be much more smooth sailing.

In fact we play that way. I'm going to try creating a dedicated server and then connect both of us to it. The problem now would be how to use the same world that we already have on that new server.

Originally posted by cubanoplay:

Originally posted by Mr_Cleene:

I have the same problem, my solution is using the devcommands just so I can repair it without a workbench so the boat can make it. It seems like it happens if you are experiencing connectivity issues between pc's. Another thing I did that reduced the frequency of this occurrence is if you run your world off of a dedicated server rather than hosting thru game directly. In this case my pc can handle running both side by side but Im sure if I had a second pc it would be much more smooth sailing.

In fact we play that way. I'm going to try creating a dedicated server and then connect both of us to it. The problem now would be how to use the same world that we already have on that new server.

i don't remember how exactly to do it but you can set the dedicated to load a certain pre-existing map on your hard drive. by default it does create a new world so keep that in mind before launching it the first time. I set that up a few months ago so I don't know if they changed the dedicated sever settings with any of the more recent updates

Also i don't know if this was addressed ion any new updates but i had to disable steam cloud or it would reset my world stuff and a few other weird thing with the saves but as soon as i disabled steam cloud and manually made backups myself i never had a single problem