i hate 420's, such tanks, and they have a daggerboard case so i always get shinbang.
I suggest you wear/bring swim shorts, if it's warm wear a rashguard (sunshirt) and try to get some booties, either paddling or sailing booties, but if ur just starting, sandal will be fine. once you progress get closed toed boots.
last year i sliced off the bottom of my big toe when i was wearing sandals, but i've been sailing for years, and do more crazy stuff than a beginner