Swimming

always felt tired very quickly.

That's because it is such a good exercise!

Wouldn't agree with that.
Swimming is hard work if you're not very good at it.
I know because I'm formerly not very good and currently alright.

But if you're tiring extremely quickly it's most likely not because you're tiring your muscles, and more down to poor or rushed breathing.
It's not really like running where you can just work up from couch to 5k (or, say, 1k for
swimming). You need a few pointers on how and when to breath and a bit of technique, then
that'll stop you feeling like you're rushing every length just to get a moment of respite and a lungful of air.

@ric_a5
If you're near London Fields then maybe get in touch with London Fields Triathlon. Looks like they do club swims there and technique sessions nearby too.

You wont be out of your league, they're only triathletes so they'll be lousy swimmers anyway. (I say this as a triathlete)