Even at anchor a charter is for hire and "underway" (just not "making way"), thus the crew required by their inspection certificate are to be aboard, not in the water unless there's an extenuating circumstance.
This is not true. A vessel at anchor is not "underway."
From 46CFR 15.301 (a):
"Underway means that a vessel is not at anchor, made fast to the shore, or aground."