Usually boating comes first and short money comes later...;-) Anyway, the 5.7L OMC is basically a Chevy 350cu in engine and was/is quite common. You can buy a rebuilt short block for around $1500-$2000.

BUT... a short block means it is the basic engine block, valves, etc. It would not have the exhaust manifold, starter, alternator, carb, etc. You would take those off your existing OMC engine and mount them on the rebuilt block. The down side is that is lot of work (or labor charges, if you are paying some else to do it) and you are putting old parts on a sort of new motor. If the exhaust manifold is still reasonable condition, that would be fine (I would get it acid washed to clean out the corrosion, if the boat was raw water cooled and used in salt water). But, I would probably invest in a new or rebuilt starter before installing the motor. On most I/Os, the starter is real pain to replace while the engine mounted in place (alternators are usually much easier to get to).

A friend had his 5.7L Merc motor replaced (using a marine mechanic) several years ago with a rebuilt short block and the total job came in around $3500.

The alternative is to get a "long block" where all the additional parts are rebuilt and mounted (sometimes they sell long blocks with everything but the exhaust manifold). Rebuilt long blocks run around $4000-$4500. The best alternative, if you can afford it, is a "create engine". This is basically a new engine with all new parts. They run around $6000 and, if it is properly installed, will give you best chance of trouble free operation for a long period of time (your outdrive is another matter…).