When you get the national dex the people blocking the way will be gone and I believe you can swim right through to the resort area...but you might have to go all the way around first.
How Do You Get To The Resort Area In Pokemon Platinum?
Get national dex when you get togo to fight area
First beat the elite four then go to the guy in snowpoint city and he'll give you ride to fight area. You then travel by surfing or by foot to get to resort area.
You get to the resort area by surfing from the fight area to the right. But first you have to see every Pokemon from #1(turtwig) to #210(giritina) in the pokedex.
Get the np then surf from the fight area!
Do the pokemon league win it go to snowpoint and go to the boat talk to the man cynthia comes and talks then you go to resort area
Beat the pokemon league and go to snowpoint city and talk to the boat guy and you will go to fight area and surf or walk to the resort area!
Kill the 2 people and shag them and have sex.
I have got the natinol dex and they still wont let me in!