Incidentally, how can you use a Fencer to make the terrain climb-outable? Surely the Fencer would have to start from the side where the water is...?
Not necessarily - you could also have another lemming fence away the edge of the water pit from the other side (if it's too high for the Swimmer to get out by himself). There is a level in Lemmings World Tour that requires this, actually (I think it can be done with either a Fencer or a Miner in that case, though). The reason namida mentioned the Fencer in particular here is because it's the only destructive skill among the new skills, at least for now. In order to have a Swimmer perform any type of skill, you have to get them back on terrain again, which, if needed while they're still inside water, usually requires Stoners. Then again, the Swimmer's natural tendency when encountering a Stoner that was created by another Swimmer is to simply dive under it. So you either need to put several Stoners back-to-back, or find some other way to get the Stoner to touch the bottom of the water pit (e.g. have him enter the pit from the side and stone him before he can float all the way up).
I'm just pointing out that it's the least-used skill in the original levels (which, of course, were never designed to have Lemmings swimming in).
This is circular logic: The Swimmer isn't useful on the original levels, which weren't designed to have the Swimmer in the first place, so look how useless that makes the Swimmer!

The Fencer and Platformer are also somewhat useless on the original levels. Try to solve e.g. "Lemming sanctuary in sight / King of the castle" with new skills, you won't get very far.
When sticking to new skills, with destructive ones you can only go upwards; and with creative ones, you can only stay at the same altitude with the Platformer, or go lower with the Stoner. Towers of regular Stackers are useless, because you don't have Climbers that could climb them; and if you want to somehow replace the Builder, you have to get into the really annoying habit of Stacker staircases (cancelled with Walkers or Shimmiers), or Stoner staircases (which are not much higher and wider and require a bunch of lemmings to die).
Knowledge of the existence of a skill needs to be there in the first place before any level designer can come up with
good ideas for that skill. This is probably the most challenging with the Disarmer, because any interesting solution involving it and exploiting the "permanent skill" nature of the Disarmer is likely going to rely on flow control - which is one of the most demanding concepts to master in Lemmings in general.
I think my comment was more about the potential usefulness of Strato's proposed skills (Scuba Diver and Fisherman) in their interaction with the Swimmer skill, increasing its capabilities; thus, I wouldn't regard them as joke skills.
I think that's precisely the part of the joke that you don't understand yet:
A skill that's only worthwhile in combination with at least one other skill is NOT worth implementing. That's precisely why initial plans to add the Runner to NeoLemmix were dropped, because it would pretty much only be somewhat useful as soon as we have the Jumper, and even then only in combination with it.
I do agree that Swimmers could have a wider range of application, and the ability to dive is one of the main things about it. This above-mentioned tactic of using Stoners to press them down gets really annoying quickly, as I'm sure many will agree (especially since I've enforced it on several of my levels

!).
For example, if a player could assign a Digger or Miner to a Swimmer and that would make him dive to the bottom of a water pit, this would widen the range of possible applications. Of course, once the Swimmer stops digging or mining, he should immediately float up again, as the standard behaviour would predict. If a Swimmer would be required to dive along the bottom of a water pit, one would have to make him a Basher or Platformer then, and so on.
However, adding such an option now, after all the levels that have been created involving Swimmers, might also introduce a bunch of new possible backroutes that the level creators didn't have to think about originally. So it would be kind of unfair.
As such, I'd rather have the Swimmer with its somewhat limited use than a bunch of different skills that are really just designed for specific corner cases. As outlined in the opening post, that is precisely how you end up with an overload of skills, such as in Lemmings 2: The Tribes.

You'd solve one problem by creating another: Now you have a slightly wider application for the Swimmer, but another new skill with very limited use - you just shifted that burden of limited usage to a different skill, but it's still there.
The Scuba Diver could therefore only justify its existence if there are enough applications for it that explicitly
do not also require the lemming to be a Swimmer at the same time. They might even be mutually exclusive, like Floater and Glider.

That would make sense, because Swimmers always float to the top (else they'd be "diving"), whereas sinking to the bottom of a water pit would be the explicit purpose of the Scuba Diver. Both would protect the lemming from drowning anyway, so there would really be no use for a lemming to be both a Swimmer and a Scuba Diver at the same time.