Hi Anguz,
I'll hazard a guess that 80-90% of the non-chronograph reps with screw down crowns (that our dealers sell) are "waterproof" and you can swim with them. But because they are reps there can be leaky crystals, loose casebacks, etc.
There's one good rule: Don't take your watches to water. Gens or reps. There are many horror stories in the gen forums how water has ruined manyy very expensive genuine diver watches. Just don't do it... or at least check your watch in a watchmaker before you swim with it.
Good luck.