Thank you so much for all of you kindly let me know.

Skeletons can enter water, just like Pirates of The Caribbean and smash a boat from below. I lost a boat that way.

If you leave your boat somewhere place a standing torch. It scares off regular greydwarves, greylings, and most animals. It also makes a 20m no spawn zone.
Also try to park next to large rocks and such so your boat is further from the shore and somewhat obscured from view.

THe easiest way to avoid this is to always break your boat down rather than just leave it sitting somewhere. It takes very little time to break it back down to it's constituent materials. If carrying them is too much, make a chest on the shore and put them in there. Then when you come back you will always have your boat.

These are all good recommendations from folks that have lost their boats and learned the hard way.
I only build Karves. They take less material, easier to drive, better for rivers-my favorite. I bring a pick axe always and chop a little boat ramp so it doesn’t float away. I build a little fence around it, like 60 wood, this works for me. And yea always have a pre portal built and portal mats ready when you get somewhere

If there’s a little island or peninsula you can cut off that’s a good place to park

I have lost a couple of Longboats like this :( One time I got back just in time to watch a bunch of Greydwarfs smashing it to bits... I swear they ran off laughing as if they stole my hubcaps when I showed up

you left your boat in the middle of the wilderness unattended, what did you expect would happen?

Bring a portal next time ;P

sounds like a enemy destroyed it whild ur underway...
if its not that long ur buildingparts should be close were the ship was last seen...
if not build a raft if no materials availible drunk urself, and respawn in the bed, or if no bed a the ritual place.. build a new boat and catch the corps...

lesson nr1 never leave ur ship alon in a place u dont know, if u go ashore use a close by isle, never go on land on the main landmass, allways keep distance or if u do stay close and eyball ur boat, they attack it as long ur in the Proximity if u leave they stop...

I've learned if you park your boat too close to the shore, then monsters will come and attack it.. so, even though swimming (early game) is a possible death sentence.. you still have to park your boat about 20m from shore.. The Karve is about 20m in length (as an estimate). Some monsters will still throw rocks at the boat but they get bored of that after a bit. Also don't park it in shallow water. Make it so you had to swim a ways to get to shore, so monsters too will have to swim to get to it. Just something to consider. I have both my Karve and my Long Ship parked about 30m from my house. Only reason they are so far is I have my swimming skill up to about level 40.. (Swimming: Another thing you'll want to level up if you plan on having and keeping boats).

tip: dont get in a boat without materials to make a portal, with a portal already at home ready to connect.

Rocks underneath may have smashed it if there was high waves/storms. Learned that the hard way. The boats can really bob up'n'down and get wrecked fast. You may get lucky if you can swim over the resources to retrieve them from the shore bottom if they are visible (shallow water) ... or you may have to build a high-dive platform to get your player to go deep enough to get them. Saw some Youtube Vids show this and it works to a certain depth point.

And as stated above, always carry Portal resources.

Jump into the water where it was. Everything you need to build a longship is still there, some of it may have sunk.

Time to build a raft.

An enemy might have seen it and attacked it while you were away. You might be able to find another biome to farm leather scraps in and build a raft to get back home. Or make a 2nd world and load into it with your current character and get leather scraps from the boars at spawn. Then log back into the world you are stranded in and build the raft that way.