I wouldn't. You don't know if they have diseases or not (they probably do). If they are in a harbor where you keep your boat (and I would assume that others do too), they probably have adapted somewhat to being able to deal with gas, oil and other polutions in the water. You don't want to bring that stuff into your tank. If you want to feed some live shirp or fish, your best bet is to buy some from the store (don't feed the store bought ones), and raise them yourself. That way you can be fairly sure that they are healty and safe.