Shoal Cave's tide depends on the time you set on the clock at the beginning of the game, which is static and unchangeable if the internal battery is dry.
Which times align with each tide can be found on Bulbapedia, but it is easier to remember this way: if the hour hand points to the top half of the clock, it is high tide; otherwise, it is low tide.
For Gen Alpha folks who can't read analogue clocks: the hour hand is the smaller one, so this picture indicates high tide. (Credit: Serebii)
Importantly, Spheal can be encountered by walking in Shoal Cave regardless of the tide, so you don't have to worry about that. The conventional wisdom is that you go for low tide, which lets you access the ice room containing NeverMeltIce and TM07 (Hail), and is the only place you can encounter Snorunt.