In that location get a stainless steel bolt/ aka machine screw.

that's a button head allen metric. more a bike shop thing, cap screw
which is not rounded more common ..
in that location,8mm hex bolt, 5mm thread (probably) would work too..
get a stainless flat washer also..

stainless is common here as that's what you want on your fishing boat,
on the salty sea.

Multiple choice, not much clamping force to hold on that cable guide,
a regular slot/phillips screw will do..

just not too long, or to take out the BB you have that screw getting in the way.