What year is the boat? Being a newer engine model with a starting of 1A*** serial number, some Mercruiser engine models have started using a separate 15 AMP accessory fuse sometimes located on/near the main wiring harness cannon plug connector that may have an adaptor type plug harness connected inline to the engine as well. Being the 15 Amp fuse is connected to the red wire which is one of the main power wires heading to the ignition key, when the 15A blows, it interrupts the power to start the engine like you've experienced.
Being an accessory power supply fuse of only 15 AMP, you have to be careful with what device you connect up at the helm to power up other accessories such as an air pump to inflate inner tubes, rafts, etc. for towing, or hand held lights for night use, etc. etc. as those items can be a big draw on the power circuitry.
Before the newer 15 Amp circuit came out, the main power supply for the engine relied on the 20 Amp fuse which should still be located up either up under the dash next to the key switch area....< Should your boat model not have a push button type circuit breaker next to the ignition key.
Some theories could be possibly an extra load of an heavy accessory item could weaken the 15A fuse while heat, vibration and time of the engine running is blowing the fuse later on....If not right away due to possibly you used an accessory with the engine on, blew the fuse, shut down engine, and get no restart. Something of that nature.
The engine never died out on you, just wouldn't restart correct?
How did or how long did it take you to find that hidden fuse? Took us 61 posts to find this hidden fuse
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http://forums.iboats.com/forum/engin...01#post5866601
If none of the above theories of why the fuse seems to be blowing on you seem to fit, then just recently there was another topic on another forum where the member was questioning why his engine would crank over on it's own without the ignition key turned... It was on a repowered 2008 Mercruiser engine model. Turned out the cannon plug adaptor wires internally became corroded away making contact with both 12v power and Yellow/Red starter wires due to these adaptors aren't as sealed up watertight as you'd think they should be.
So you may have a loose connection someplace along those two wire circuits of the hidden fuse causing excessive heat blowing the fuse or corroded wiring within the cannon plug connector causing issues intermittently due to design.
All that mentioned, you're roughly the 4th known forum member to me questioning this fuse one-way or another. It could be just user error as described above or my personal thought is it should of been a 20 Amp fuse from the factory to match the 20 Amp fuse up at the helm. It's rare to see, read or hear of the 20 AMP fuse blowing for little reason...Anything smaller installed seems to blow for little reason...Just saying not advising.
Somewhere I believe to have recalled seeing Mercruiser offers a bigger Accessory fuse adaptor kit at I believe was 40 Amps.: Small update Search online 84-889611K01 - Power Harness Kit which is the 40 amp fuse and uses 2 bigger red wires,
http://thumbs4.ebaystatic.com/d/l800/pict/112196703746_1.jpg
Below is one image of the possible adapter designed used with the hidden fuse + the adaptor showing the wire corrosion,
