I held back from answering your original question as I was wondering what the source of it was. Now that I know that it's an application form, I'll add my opinion to those you've already received.
I think that everyone has agreed that increasing speed is not a good idea. So I think that the correct answer has to be based upon 'reasonable assumptions' about motorway driving.
In general, you don't come across really deep water on a motorway unless you've already gone through some significant amounts of standing water first. There's nothing in the question to suggest that you've already been driving through water, so it seems reasonable to assume that it refers to finding the first bit of flooding. This is unlikely to be so deep that you'll suddenly sink to your doom, so it seems sensible to continue driving slowly through it. (With reference to your subsidiary question, above, 'reasonable assumptions' make it unlikely that any really deep holes are going to suddenly appear in a motorway. Have you ever heard of it happening?)
Further, if you drive into a flood which is deeper than you think, the worst that can happen is that your car will come to a halt and you'll have to wade out. (Any following cars will either be going slowly or also be brought to a halt). However, if you stop at the edge of the water to get out and investigate, the worst that can happen is that a following car will plough into you and kill you.
For those reasons, I'm confident that the correct answer is to proceed slowly through the water on the road.
Chris