Jellyfish, sometimes 1 foot diameter, sometimes only 3 cm or so.
The small guys only make a low pitch rumble as you kill them, but the bigger guys front flip you fast.
Of course algue also, sometimes so much you need to stop flying and flip your foil up to hand remove!
Sometimes you feel the strands on the strut , but by landing and relaunching you can often make them slip off.
One time I passed well down wind of 3 fishing net markers floaters only to find my foil tangled in a metal reinforced line.
This took 3 upwind walks and body drags out to get it untangled.
And of course repair of my strut le and te!
Twice I got fishing line well wraped around both strut and wings, had to body drag in with foil as it simply would not fly with all that shit on it!