A friends of mine can`t keep his port engine running. Starts and within 10 seconds it peters off. But he can run that engine on the hose and it`ll idle for 5 minutes without skipping a beat . puts it back in the water dies , runs it on the hose at the launch , no problem, 5 minutes later it`s in teh water , won`t run.
Make no sense to me.

Is there a way to see IAC count TPS position, do they need a TPS reset ?
It has a stage 2 kit and he said the ecu might be locked .. when it`s locked no parameters can be seen?

Never seen that with through hull exhaust, but in applications with through-prop exhaust, if the engine is barely running because it's worn out, low compression, not firing on all cylinders, etc, then it might be able to run on the hose, with no backpressure against the exhaust. Drop it in the water and now the exhaust has to get pushed out through the water, motor won't run. But with through hulls that wouldn't make any sense as an explanation.

Also, is there enough fuel in the tank that it's not sloshing away from the pickup when it's in the water versus at the angle on the trailer?

A ‘locked ecu’ just means you can’t go in with software and see the programming nor copy it.

You can still check live engine data and for stored codes and etc.

What kind of scanner would he need?

Rinda
https://www.rinda.com/rindashop/shop/techmate-pro/techmate-pro-94706

I wouldn't own a modern engine having a scanner handy. He will also need the dongle for that engine.
https://www.rinda.com/rindashop/

Is it beeping?
IAC, TPS, Crank, and Cam Position Sensors would be things I would check for faults. The latter two can sometimes be bad without triggering a code as well.
Does it have plenty of water coming out of the tailpipes when it's in the water? Does he have water pressure gauges? Does it have hose connection, or when it's on the hose, is he using muffs?

this should work make sure you get the correct adapter for the pcm..........ooops dup post ^^^

https://www.rinda.com/techmatepro/

Without a scanner its like shooting with blindfolds on.

Long shot....I had an issue like this that turned out to be water in the fuel tank.
On the trailer would idle fine- at a bow high attitude on the ramp or in the water would sputter and stall.

oh crap, thats funny you say that , his fuel seperator had water in it and I saw bubbles in the actual filter .. we dumped it out put fresh gas in ( the fuel filter) and it ran fine for a minute till it started again. we thought we had it .

I had basically the same thing happen on my 525SC in my Pantera after the winter layup. I had to dump my fuel filter about 3 times