Tides are the constant rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Sun, the Moon, and the rotation of Earth. Why does this matter for surfing, you ask? Well, because understanding that water depth and tide cycle affect how and when waves break will help you know when to surf. It will also tell you when not to go!
Knowing about the specific surf break that you’ll be surfing is also important in planning when you want to paddle out. For example, a lot of reef breaks tend to work best during an incoming tide, or as the tide rises from low to high. Surfing during low tide at a certain reef break may not even be possible due to the exposure of dry reef or the water depth just might be too shallow and dangerous. In contrast, high tide at another surf break may fill it with too much water resulting in swamped out surf (fat or mushy waves) that break right onto the shore resulting in unsurfable conditions.
Various factors including swell energy, water depth, and whether the tide is incoming (low to high) or outgoing (high to low) will affect wave quality and ultimately, surf conditions. Since the seafloor topography (bathymetry) is unique at all surf breaks, the tide will affect each and every break differently. In general, waves will tend to be steeper and faster during low tide yet mushy and slower during the high tide.
You’ll need to familiarize yourself with your local surf break (or the surf break that you want to surf) and how the varying tides and water depth will change wave shape and surfing conditions. The best way to know is by gaining more experience and time in the water while always being aware of the swell and tide data. Sooner or later, you’ll learn the ideal tide range and know exactly when to paddle out and also when to not bother wasting your time looking for surf.