"aren't most all alternators and starters sealed?"
No, they aren't. In a car or truck, there's no need for this and if the Coast Guard inspects your boat, finding a non-CG alternator or starter, they will fine you and tow you in.
"but aren't there dozens of potential spark sources in an engine compartment, thus you should always run your blower before cranking?"
'Should' and 'do' are two totally different things and when someone is "in a hurry", safety measures go out the window. Ever seen a boat that burned or had an explosion? They go up fast. If you need a starter or alternator, a good shop that rebuilds them will make them conform to CG spec.