I think it is sometimes hard to know what is unreasonable paranoia and what is just good common sense in the light of what we know about blood-borne viral infections. Everybody has to draw their own lines I guess. The principle is that blood-to-blood contact is a no no.
So, common sense dictates to stay clear of another person's blood wherever it might be found, and that goes triple if you have any open wounds on your own body. So for the fish in Thailand I'd be asking myself - could the person before me have bled into the water? Did they change the water between people and disinfect the pool? How long could the various viral infections live in the water? These questions have answers, so asking them and staying informed on the answers is the best defense.
It has often been asked if mosquitoes can spread HCV, but to my knowledge there is no evidence to suggest that they can. I have heard it suggested that contact sports where blood is drawn by both parties, can be a risk. If blood-to-blood contact occured then I would reason that that is a possibility. Trying on somebody else's earrings or piercings might also be a risk. Any accident situation where somebody is bleeding, no matter how small, is an automatic risk, especially if there's broken glass which could cut you.
I'm no authority on this subject. I'm just trying to step up my thinking about it in a way that I admit I never used to think before being diagnosed.