We've had bath crayons, they've been super easy to wipe off. The first set we were given and don't remember the make, the second set was from the £ shop and the quality was just fine, still washed off easily. TBH we have has issue with plastic toys (a couple of boats) which have left red marks all over the bath, the crayons were nowhere to be seen when the red marks appeared, it was definitely the boats. Those marks took quite a bit of scrubbing, or rather scraping off with my nail. (they were Thomas the tank series, Bulstrode I think the name is...had no problem with a cheap set of plastic stacking boats we got) What I found is the crayons didn't last super long, if you leave them in the bath water they will go too soft and possibly break, try to have a pot or something to pop them into rather than dropping them in the bath to soak iyswim
WRT squirty toys - I think there just comes a time you stop using them to avoid the mould! We have solid plastic toys instead (things like octonaughts characters, tub of farm or safari animals (you could get fish, we don't have many fish though), stacking cups/boats are great. Also a set of 3 stacking funnels (from kitchen department £ shop), syringes, little plastic bottles, spray bottle, droppers, washed out tubs/bottles such as squeezy ketchup or washing liquid bottles. Bath letters and numbers have been super fun here too, they are foam and stick to the side of the bath so you can write names or use them to draw funny faces with, do colour matching etc. Bath books are fun too.
