Yea, just because something is water tight and buoyant, or at least had controllable buoyancy (either through ballast or contra grav) is a completely seperated issue from whether the ship is efficient as a sea going vessel. One doesn't imply the other. Oil platforms, for example, are sea gong vessels, and spectacularly bad at it. But they're there nonetheless.
When folks mention fuel skimming, I think folks think of things like fire fighter water bombers. But that's not what come to my mind at all.
Rather, the ship lands in the sea and parks, and then opens up the appropriate valves and starts taking in water. It's not a fly low and scoop operation. It's more like dipping.
As to the earlier question about unstreamlined craft landing in atmospheres, it's a valid point.
If a ship has, essentially, zero velocity, then its streamlining shouldn't matter. Use contra grav to slowly and gently lower the ship in to the atmosphere.
So, technically, I think it could be done. But I don't think it's practical.
The problem is simply winds, and a ship, even one with a ground velocity of 0, would have to contend with winds. For example, in the earth upper atmosphere the winds are over 200 MPH.
So, since a ship simply isn't buoyant enough to be blown along the winds like a leaf, and thus maintain 0 relative velocity to the winds, there's going to need to be some kind of streamlining I would imagine.
Now, to be fair, you'd like to think that a ship that can withstand 6gs of acceleration, regardless of configuration, should be able to withstand at least some amount of wind force.
But using the classic "big dish" antenna, I think that this is something that could withstand 6Gs of acceleration, but the wind may well produce more force than that.
So, anyway, I think it's fair to say that "no", an unstreamlined ship can not land in an atmosphere, at least without some damage to the ship. I can certainly see it being an interesting role playing event, and I would think that a ship should be able to land reasonably intact, particularly any of the major ship configurations other than Dispersed Structure, and the ship may even be able to take off again later. But it will be a slow trip, both coming in and getting off, and filled with peril.