The point about EPS is that it has lots of open interstitial spaces, which do fill with water under pressure/immersion - but the important bit is that the water is very free to drain out again when pressure/immersion is removed. The other important bit is that EPS is low on capillary action - it doesn't significantly 'wick' water up.

So, immersed in sea or any water, EPS becomes waterlogged - no doubt packaging EPS has more interstices to fill than 'graded'. But no 'grade' is going to resist waterlogging.

However, thermal insulation holds up fairly well even when waterlogged.

EPS won't become waterlogged, even when standing in a puddle, above the 'water line' - it doesn't appreciably wick the water up, tho it may seem 'soak up' liquid water on its surface e.g. rain but only if it's driving rain creating pressure by impact.

When removed from the sea or any water, most water will drain out freely, leaving interstitial surfaces wet, which are then free to dry out by evaporation.

An EPS surfboard would indeed become waterlogged if its skin were cracked, being immersed and/or under pressure of impacting water, and its skin being a closed 'container' (apart from the crack) would no doubt be very slow to drain out, even slower to dry by evaporation.

Open-matrix EPS is to closed (bubble) cell plastic foam insulants as soft breatheable lime render is to hard, supposedly impervious cement render.