You can always be sued but that does not mean they would win. I don't think they have a case. Since we don't know how the finger got broke it is hard to make a judgement... I am guessing that the pond is constructed as an ordinary pound would be. If the guy broke his finger on your gate, boat dock, shade umbrella etc... then you may have liability. If a snapping turtle crawled out of your pound and broke his finger you may have liability. If he slipped on an established walk way you may liabiltiy. If there are no caution signs posted you may have liability. I think you are ok to "wait and see". It would be good if you could record statements from any witnesses as to how the finger was broken... That is what it all comes down to. Did your allowing him to fish on your property cause his finger to break?