Took out my new boat yesterday,well new to me anyway and it ran fine for the first couple hours.Then all of a sudden it stopped running.Thought it was the starter,but it's not.Took all the plugs out and cranked it over.Water gushing out all 8 cylinders.How could that happen and is the motor "toast"?
It's a 1999 502mag mpi.What's my next step here guys.

Boy this blows.Brand new boat and roasted motor.

running at speed and it just quit ? Or just comming off plane and it stopped ? need more info ....

What kind of exhaust?

cracked block

We were running at speed and the engine seemed to be choking and the alarm somewhere in the engine compartment when off and the motor stopped running.

Here's some more info if it helps.Did a compression check.Each cylinder is 160psi which is pretty good for our elevation which is 5280 above sea level.Blew out all the water in the cylinders,put new plugs in and it fired right up using the garden hose attached to the lower water pickup.However,the oil looks like a milk shake.I will be changing the oil now....Does all this indicate a cracked block or cracked cylinder head?

I chased water in my oil last season. Finally took the intake off and found cracks in the top of my cylinders in the water jackets... sucked.

Well................. I would say block, exhaust manifolds, or Head gasket.

sounds like a head gasket, or cracked head or hole in head, if it was an exhaust manifold the oil would be fine , now that I am thinking of it since the compression was good in all cylinders I bet the intake gasket is blown that would do both things for sure

Water is in both banks.
Look at the common connection here tying both sides together.

Stainless risers or cast iron? Huge problems with stainless risers,they leak at the gasket surface & into cyls.