Reading a surf launch
Watch the break for several minutes before launching, wave sets come in groups with calmer gaps between them. Launch during a lull, angle your bow directly into oncoming waves rather than side-on, and keep paddling forward through the break zone rather than stalling.
Rip currents
A rip current is a narrow, fast-moving channel of water flowing away from the shore, usually visible as a gap in the breaking waves, choppier or discoloured water, or a line of foam/debris moving seaward. If caught in one, don't paddle directly against it, it's usually narrow: paddle parallel to the shore until clear of it, then head back in.
Lifeguard flags and patrols
RNLI and other UK lifeguard services fly a red-and-yellow flag over the patrolled swimming/bodyboarding area and a separate black-and-white chequered flag marking a zone for surf craft (including kayaks/SUPs) to keep clear of swimmers. Patrols are seasonal and daytime-only, not present at every beach or at all times, check locally before you go.
Longshore drift
Waves rarely arrive dead straight onto a beach, most break at a slight angle, pushing water (and anything floating in it) sideways along the shore over time. A calm-looking launch spot can still drift you along the coast during a session, useful to account for when planning a return to your original take-out.
Offshore wind on the coast
A wind blowing from land out to sea is far more dangerous close to shore than the same strength would be inland: it can make paddling back to shore exhausting or impossible, especially on a SUP, and typically gets stronger, not weaker, further from the beach. Treat any offshore wind forecast on a coastal route with real caution.
What this app doesn't cover yet
Licensed nautical charts, real tidal stream (current) data, shipping lane/ferry route information, and AIS vessel tracking all need paid, licensed marine data this app doesn't have access to yet.