Modelling assumptions: Mechanics

So I was doing Q4:
https://ibb.co/PxFWfw6
and Idk if my criticism towards the model is correct.
My answer was:
The boat would be affected by drag in the water, so as an external force is acting on the system, the momentum will not be conserved. I understand the reason behind the actual answer, but I was wondering if my answer would be correct.

The actual answer is:
The boat is large and heavy, so there will be additional tilting/rolling motion. The boat is also on
water, so given waves, tides and currents it is unlikely to be at rest initially.

The question talks about "immediately after diving" so drag will not yet come into it, so, no, your answer wouldn't be correct.

I think the given answer is a load of *******s. Could be a rowing boat on a lake - so large and heavy, and tides and currents don't necessarily come into it. The only reasonble objection there is the tilting rolling motion, IMO, also no account is taken of vertical motion in the dive; it would be virtually impossible to dive horizontally.

Right, I see what you mean about the drag part ( and the vertical motion part). Also, how would the tilting/ rolling motion change the answer?

Total kinetic energy is the sum of linear kinetic energy and rotational kinetic energy. The latter relating to the tilting/rolling motion.