They're not in long enough to develop any serious pressure difference in the hull.
Hot day / cold water. Nearly instant pressure change once the boat hits the cold water. Reverse is cold water sailing and then leaving on the hot beach.
Mmmm, not quite, since the air in the hull needs time to cool - and it's in an insulated container (foam core). Air itself (especially 100% humid air) is a pretty good temperature moderator, since the water phase change soaks up/bleeds off a lot of energy.
I've never seen a vacuum drawn on the hull, when I pull the plug - only overpressure.
But Matt's right - extreme temperature changes (say 40+ degrees F) will create a significant pressure differential. Lemme dig out my old thermodynamics book and I'll figure out exactly how much.
I even remembered the Ideal Gas Law (PV=nRT) before I got to the thermo books. Since V, n and R are constant, P2=P1(T2/T1).
Extreme case - going from 50°F (283.25 Kelvin) to 120°F (322.04 K).
Assume P1 = atmospheric pressure = 98 KPa
P2 = 98 KPa(322.04K/283.25K) = 111.46 KPa
ΔP = P2-P1 = 13.46 KPa = 1.95 psi
2 psi is borderline blowing the boat apart.