Skull and bones pretty much only looks like ship combat. While sea of thieves has exploration, guests, map hunts and ship to ship pvp/pve.
So.. Similar but ultimately, no.
Yeah, I just looked at both the make sure I don't have an anti-Ubisoft bias. I don't. Skull and Bones looks incredibly shallow mechanically. Sea of Theives absolutely doesn't.
What the latter is comparable to is Lovers in a Dangerous Space Time. Mechanically, it's that game but in 3D and greatly expanded in scope.
The only disparity is that Skull and Bones looks better graphically, which as far as I'm concerned is completely irrelevant. Gameplay is king, and SkullnBones looks terrible in that regard. Sea of Theive's, by comparison, looks amazing. It's just not flashy.