You would not believe just how complex this sewing thread issue can become. Seriously!
One major use for polyester-core, cotton covered (poly-cotton) thread is in high-speed stop-start machines. The high-speed bit means the thread is screaming through the eye of the needle and the needle gets hot. When the machine stops the hot needle can actually partially melt a straight polyester thread. The ensuing tangle and disaster has to be seen to be believed. The needle usually breaks and the broken end punches big holes through the fabric. $$$! But when the thread has an outer cotton layer (eg Amann Rasant) there is less friction (so less heating) and the cotton surface means the thread does not melt either. Everyone is happy. What relevance does this speed business have to home sewing? Not much.
Yes, the cotton will swell slightly when it gets wet. But the amount of swelling with a light poly-cotton thread is so small it is extremely unlikely to do anything about filling up the needle holes in my experience. However, PU and silicone sealant do stick fairly well to the cotton surface, which makes seam-sealing a bit easier.
A straight polyester thread (eg Amann Serafil) can be very smooth and strong, but it can also get fouled up in a domestic machine. The surface of the thread is not really smooth and the thread tends to get twisted as it goes through the friction device. The end results are tangles and jams in the thread before it gets to the needle. These threads can be used, but you have to know how.
For sewing packs we often use a very strong bonded nylon thread. These do not twist up the same as the 100% polyester threads (fortunately), but they are all too heavy for use on clothing or tents.
I am not all that enthused about the mass-market (Gutermann etc) threads, at least not for tents etc. But I do use those brands for Taslan (Supplex) clothing because I can get matching colurs fairly easily. Only a few shops carry the Rasant thread, and then in only a few colours in the 120 size, because the Amann brand is more for commercial use. I am not sure it has much penetration into the USA market, but then little commercial sewing is done in the USA these days.
You can also get big cheap reels of overlocker thread in local shops. This has a very slow twist. It's fine for overlockers, but the stuff can be a disaster in a conventional domestic machine. Don't even try that.