Any advice on the easiest method of removing tide marks from the wall tiles in a pool, those above the water level?  

If it’s a hard whitish coating 2-3-4 mm thick its calcium, use a Stanley knife blade, scraper or sharp wood chisel to gently but firmly scape it away then use something like CLR clear and/or a phosphoric acid based brick/stone/concrete cleaner and a stiff brush.

Less than that it’s likely just scum so maybe just the phosphoric acid cleaner with a nylon scourer (don’t forget rubber gloves) 

You'll probably need to at least lower the water level.

 

80% water 20% hydrochloric acid in a spray bottle and rubber gloves + elbow grease. Wont have to lower the water level.

Can get the acid at any pool shop.

 

Good luck.