If you have a leak, it'll be pretty obvious where the leak is to dump that amount of coolant that quickly. Look for wet/greasy spots. Specifically look at hoses and fittings (don't forget about the aux fitting coming out of the back side of the head under the intake manifold!).

If you don't have an obvious leak (a leak that large will be obvious), go look at the exhaust. If you see white smoke or steam, you have a blown head gasket or a cracked head, both are fairly common on the OM603.