electrical issue on my 4.3L gas engine

[GerLech55]

Hi
Problem with my Glastron GX185 from 2004
Engine is a gas Volvo Penta 4.3L

Worked perfectly fine until a few days back the engine stopped working when at full speed.
Since then, the engine won't even crank.
Battery is fully charged; what happens is this:
* at first I can trim down
* the first time I turn the key, it seems like the starter goes, then it makes a squeaky noise and ... nothing left, cannot even trim up, like there is no power left.

I know nothing about mechanics but this seems like a bad relay or something (I checked the fuse on the battery line and it is good). Any tip for an easy fix (dreaming cost nothing, right ?) before having to tow it to a pro?
Many thanks in advance

 

[alldodge]

Worked perfectly fine until a few days back the engine stopped working when at full speed.

So your running down the water and the but dies dead in the water?

How much oil is in the motor and what does it look like on the dip stick?

Battery is fully charged

How do you know it is fully charged, did you use a voltmeter?
If volt meter was used, what voltage did you read?

Connect the meter to the battery and push trim, what is the voltage reading change to?

 

[Warrior By The Water]

Check your negative terminals. Sounds like bad ground