StormJib said:
"For check-in: 180 minutes (3 hours) prior to departure"

That is to factor for the time it can take to get from the curb through the lines of the day to the gate to board. Each Day and Time of Day is Different. Sometimes SST can be a bit of a zoo.

"For boarding passes or to check bags: 90 minutes prior to departure"

That is the point where the computer may lock a traveler out no matter how long they have been in line. During periods of heightened security, that decision can be made by a government 1,000's of miles away.

What worked for one person ten times has no relevance to any future group trying to get to their seat on the plane before the computer closes or the plane door closes.

Putting the ferry in your critical path has a history of leaving inbound travelers in STT the first night and getting outbound travelers to St. Thomas too late to make their scheduled flight. When that happens the burden is all on the traveler to sort out. Many do just fine with the STT and the ferry combo. Many end up very disappointed. Yes, It is your vacations and emotions. +1 on "Friends do not let the friends ride the ferry"!