One rating for getting out on the water in anything small: a jet ski, a fishing boat, a kayak or a paddleboard. GoDay scores every daylight hour, then finds the best 2-hour window of each day.
Also on the page, but not in the score: wind direction, tide (a model estimate), moon phase.
A thunderstorm holds the score at 2, and one due within the next two hours holds it at 4. Gusts over 60 km/h (37 mph) or fog under 1 km hold it at 3.
Daylight counts too: at dusk the score is held at 5, and once it's dark at 2.
Under 10 km/h (6 mph) is glassy. Up to 20 km/h (12 mph) is still good, up to 30 km/h (19 mph) gets choppy, and past that GoDay calls it rough. Gusts count too: past 40 km/h (25 mph) they pull the score down fast.
Half a metre (1.6 ft) or less is excellent, and up to 1 m (3 ft) is good. The wave period matters as well: short, steep waves under 6 seconds make a bumpy ride even when they're small.
Yes, as information: a sea-level model's estimate of whether the tide is rising or falling, plus the moon phase. Neither changes the score, and neither replaces local tide tables.