integer index =llListFindList(boatcreators, (list)this_creator);
    llOwnerSay((string) index);
    llOwnerSay((string) this_creator);
    llOwnerSay(llDumpList2String(boatcreators, ", "));

outputs:
[17:22] Bandit 50/3 Trailized v1.01 S1: -1
[17:22] Bandit 50/3 Trailized v1.01 S1: ee77f966-07a6-417b-b002-036c75d21119
[17:22] Bandit 50/3 Trailized v1.01 S1: ee77f966-07a6-417b-b002-036c75d21119, 1240516c-7ee7-4f1d-806b-1448e86248d8

-1 = not found, yet its clearly in the list..