SCREWS

How the footrest and lines hare connected to the deck of your surfski?

In my surfski they are just screwed, I dont like it, it's weak.

It would be better something like this, the female brass screw glued to the deck.

Maybe replacing them with well nuts might be a good alternative - I've done that on my rotomolded kayaks but I'm not sure if they are appropriate for composite boats.

I think you'll find (at least in the case of the Stellar) that the screws you've highlighted are in fact metal threads mated to 316 stainless nuts moulded into the fibre-glass.  Certainly that's been the case on every Stellar I've modified to incorporate a barrel-adjuster for the rudder lines.  Easy way to check though, - just unscrew the fastener.

This combination is undoubtedly stronger and more corrosion-resistant than the stainless/brass fastener combination you've suggested.

I have to find a way to solve this issue. In my ski everything is screwed in the composite thin layer, ever the footrest.

very weak and unsafe.

Matteo you seem to have invented a problem no one else has. All my fenn skis, think skis, revo skis, epic skis. Carbon, glass you name it simply had stainless threaded screws into composite. The screws are placed in locations where they will experience a shear force and not a tensile force (pull).

If you are pulling or shearing screws out of the composite you are either abusing or incorrectly using your ski. Put the screws back. Add some epoxy if you need to. Stop worrying and go paddle. These boats are stronger than you think.

Not sure which from which ski your image has been taken, but here is a photo of the type of screw Stellar use:

Yes, there are  nuts moulded into the fiberglass in your Stellar , as you told me.  That's a huge difference .  

matteo wrote: I have to find a way to solve this issue. In my ski everything is screwed in the composite thin layer, ever the footrest.

very weak and unsafe.

Am I seeing things or is there some sort of a sleeve or anchor int that hole?

To echo others...I would just inspect how solid are the connections. Maybe paddle in a safe area and push the boat around a bit and if everything holds fine, I'd just let it be. 
(And believe me, I am not a person to let a loose bolt untightened).