We are about to go on holiday where there's a 'small pond' in the garden.

[BrownPaperandString]

...and we have a 3 year old and a 2 year old.

I've only just noticed the pond bit.

Dh doesn't think that we need to be physically looking at the kids all of the time.

I think we will need to for obvious reasons.

Now that I know he thinks what he does, I'm not going to be able to relax at all now am I

You are right and your dh is wrong, wrong, wrong. The trouble is, however much you shout/argue/show him things, I doubt you'll change his mind (though he may say stuff to humour you) so you'll have a seriously unrelaxing holiday.

I would investigate options for covering the pool temporarily. A covering, or a big playpen/stairgate round it, or stakes and chickenwire. Something.

Try and get a rigid cover as the accident I referred to was a small pool with a cover on, she fell under it and couldn't get up (it's thought)

We moved into a house with a pond last year when my ds was 3. We fenced it off so there was no way he could physically get near it. Now I am paranoid that he is a bit older will climb and get in that way - so I never allow him in the garden unsupervised.

This is meant to be a holiday for you - how will that be restful if you can't take your eyes off them? Tell your DH to cop on - and if he refuses to take on board the risk to your dc's - change the location of where you are going or pull the holiday.