My wife flew in and out of St. Lucia several times carrying her own expensive meds and both of us flew in and out of St. Martin. They were not in the least worried about prescription meds and didnt even search our bags or anyone we ever saw. As above, they have rules on booze and cigarettes, so obey those and you should be OK
In general, all these islands make their living from tourism. They want you to feel happy and welcome. They are not into the police state thing. Sure, some of them have silly bureaucracy for boaters but none of that applies if you fly.
And as for customs
for boats, we were in and out of most of the islands all the way down the chain to St. Vincent and NOT ONCE did any customs guy set foot on our boat. In fact, they usually didnt even look out at the bay to see that there WAS a boat. Sometimes they didnt even have a view of the bay. The closest they ever came was in the Bahamas where the nice customs lady asked us to come sit at a picnic table in the shade to do the paperwork where it was more comfortable.
If you smile and try to follow their rules they won't give you a hard time.