Deep Sea Search Question

Can someone please help with this? I do not understand the round timing. If the player is allotted 5 minutes (not sure of exact amount), at what point does the level close if not all items are found? And- at the final level, I get a 30 second warning and BOOM!- the game ends immediately (even if I still have items to locate) and I don’t have the option to use the fully allotted time. I get that unused time adds to the overall score, but shouldn’t it be the player’s choice to finish before the time runs out? It’s so weird. I have lost so much money because I do not understand the logic- nor do I see this addressed in any of the rules.