Something is likely triggering your Rev-Limiter. How's your oil level? When you were running, did you turn close to 6,000RPM's? A fouled water pump could cause it to overheat, which would also trip the rev-limiter. When you turn the key switch to "ON" position (don't try to crank it), watch your speed and tach gauges. What do they do? Any lights flashing or illuminating continuously? Does your RPM needle rise to a certain number on the tach, hold for a moment, then drop to zero again? When you were running the boat, did you have any alarms or flashing lights go off?
If you're running it at home on the muffs out of gear, the outboard wont let you rev past 3000rpm's. It should be surging and dropping RPM's rapidly right around 3 grand. Is it doing that? Since you probably didn't have it in gear at home (backyard rooster tail?), I wouldn't factor that symptom into the diagnosis.
This engine is one big computer full of fail safe's, sensors, and limiters, so a sure way to find out what's wrong with it is to take it to a Suzuki dealer and let them plug it in for you.