I regularly have this problem on our boats at work as we share the lake with fishermen. After lots of expensive repair bills I regularly check each engine ever 4 weeks for fishing line wrapped around the prop shaft in front of the prop.
If left undetected it wraps tighter and tighter and gets drawn into the gearbox through the seals and let's water in and wrecks the oil. If its nylon line it can also melt due to friction, even under water, and be a real bugger to get off from what ever it sticks too.
Simple enough job to change the oil and shaft seals. If the milky oil smells a lot when drained you know it's been in there a while. Change the drain plugs seals while your at it at 50p each it's worth it even just for piece of mind. When you drain the oil look at in sunlight for signs microscopic metal particles which could be your gears Wearing due to the oil not functioning properly.
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Freelance skipper, RYA Powerboat Trainer and Advanced Examiner