may seem basic and you prob checked it but quite often this can be weed around the intake. (sounds like the exact symptoms) usually just put in reverse and give it a rev.

you limped back in OK under idle in gear ?

Was it woodies by any chance?

 Yeah managed to get back easy enoug and it was woodies, about 8, 8.30 ish. 

Went out again today with an empty boat on the river and same issue, got a bit more power but I. Assume that's. From reduced weight in the boat. 

Hey mate

Where abouts you located? One of my good mates is a marine mech. He could probably have a look at it for ya before christmas if you want.

Let me know and i will PM you his number.

Cheers Jayce

 In your fuel?  I had this is a jet ski. Couldn't rev it but ok at low revs. Put a additive in the fuel to get rid of the water, 10 minutes later, good as gold.

 Similar thing happen to me yesterday. Think I have a faulty injector. Can't get anyone to look at it till the 10th of January. Sux when I have a bit of time off over xmas. 

Sounds like the CDI power pack. Had a similar problem on my old 2stroke took a while to diagnose had the same symptoms but i would check compression first to make sure it isnt anything more because a blown cylinder will have the same symptoms. 

Is it flooding (with too much fuel)?

What type of engine?  If it is a modern inboard it could be blocked manifolds (engine cut out due to overheat)

Had a similar thing happen and it ended up being a hole in the water jacket. Was a small leak in the top of the cylinder. Would run fine off the hose in the back yard but under load would crumble. Ended up replacing the engine for a newer 1. Apparently happens a fair bit at the cylinder closest to the water.

 http://fishwrecked.com/forum/outboard-issues

Is it something like this^^

Have a 75 yammi 2 stroke ~ 2002. last year when ever got over 3500rpm it would alarm and derate costme a fourtune in trying first impellar , then clean out and check water jacket ect , ect , ect ended up looking at new motors but after talking to the marine mech he said it could be the sensor but had never had one fail yet i was ready to buy a new motor so told him to change it any way and it's been good as gold ever since

 It's a 90 hp mariner 2003 outboard 3 cylinder 2 stroke. All the cylinders have pretty similar compressions, tried 2 different tanks of fuel so don't think it is the actual fuel or the fuel lines but will get some additive and try that. My next step is to grab an inline spark tester and check for spark to try and Eliminate as many issues as I can. Could be a cdm coil module and that's a simple fix - bit of a google search shows exact problem with a number of causes so bit of a stab in the dark at the moment.

i feel your pain Doaks, really bad timing that's for sure. 

 But of an update for those that are interested and it may help someone in the future. Got a spark lead tester from auto shop for $15 and managed to diagnose the problem as a defective cdm module (coil and lead module) so picked one up yesterday and water test on the river today had me back to 34kn with no loss of power. Thanks for the advice from those above (anda. Fair bit if google searching) am pleased to be able to get back out again. 

 Good work getting it sorted 

I had the same thing on my bike 

nothing worse than the loss of confidence in the thing to get you home again or the thought of the cost of a new one

glad you got it sorted