Steering would be painful. Trains work particularly great because of the tracks.

Also, drag on a hull is such that longer hulls tend to be more efficient.

Would you need to do much tricky steering outside of getting in/put of ports? At ports, even ships themselves are often pulled by tugs because of the difficulty of manoeuvring in such a small and busy space.

Wind and waves would be a problem as they'd push the train around and it'd be hard to control with a tug. I think it'd be less hydrodynamically efficient too which would mean higher fuel costs overall: assuming you could build a single large ship with the same capacity as the train then it'll have a lower surface area => lower drag.

I would think (i.e. pure conjecture) that it's about economies of scale. Making enough smaller ships to make trains will costs more steel. Making a train that fills up a similar area in the ocean reduces the available storage per shipment.

Follow this thought to "let's just make these trains as long as possible." You can imagine some wild consequences, both positive and negative.

River barges basically work this way, likely mainly due to tighter curves on rivers and inland canals.

We do, I have seen three or four smaller hulls. On river

At that point you might as well make multiple ships.

Surely it's a lot easier and less expensive to build an engineless, ship "trailer" than a self-propelled ship with an engine, fuel tanks, propellers, a generator etc?

The ships are already at the maximum efficiency size given their engines, so to pull a similar sized ship they’d have to add an engine to the first one, in which case why not add it to the second and have two ships?

Could they be pulled by multiple small tugs instead? (no idea if that's feasible on the open seas!)