The saltwater launch gate stays open unless they close down all launching. Now if you stay out to late you may find a warden waiting on you when you get back. If for some reason you get back late just claim boat trouble.
Here is the rules straight out the rule book
http://www.fws.gov/southeast/pubs/SWLAcomplex_Fish.pdf
Boat admittance into the refuge is permitted
from one hour before legal sunrise until one hour after
legal sunset; however, fishing, crabbing, and cast netting
for bait are prohibited until legal sunrise and after legal
sunset.