I just heard a lecture say that we shouldn't visit the beach according to Sayed sistani due to the fact that it will be to difficult to not sin either with eyes or with someone on you with their eye on you.

Is this an actual rule or is it a suggestion and if you feel like you can control yourself from looking and you personally dont swim unclothed for others to look it can be ok?

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Yes. What you heard is correct.
S. Sistani says that even if you can control your self, it is not allowed to go based on obligatory precaution.