I'd had problems with water marks on and off for a few months when I first started.
I tried a drop (only one drop, as warned by the local darkroom supplies shop) of washing up detergent. This didn't really help much, but things improved a little when I allowed the negs to dry quite slowly in my drying room (doubles up as a bathroom).
I bought some real wash aid, this was Jessops Econowash. This too was used at the rate of a few drops per 1 litre tank. Again, this was a disappointment, with some marks here and there, usually on the best frames!
Then I read an article in a mag about washing negatives. They tried a few different makes and reported their findings. The testers followed the manufacturers instructions carefully and found that they all did a fair job. The major variation, IIRC was the price.
I thought, "what a good idea", I'll read the instructions (for the first time!!!!!).
Although I had been putting a few drops into a one litre tank, the instructions suggested it should be used at 1:500. So I used a syringe to put in 2cc per litre tank, which is much more than a few drops. That was it, drying marks are now rare on my negs. I generally see them only when I have speeded up the drying process when running camera test films.
Not for the first time, I had been caught out by simply not bothering to read the instructions. Must write on the back of my hand....RTFM.