Question

Vejam se há erros de gramática ou concordância por favor:

I spent the last few weeks analyzing and investigating the disappearance of several bathers on the beach of Santos and finally I came to a conclusion.
There were a lot of rumors talking about strong currents and waves that were drowning people in the sea, but when I talked with the lifeguards I confirmed the opposite because in fact the tide was very calm.
Searches for bodies were made and nothing was found which led me to believe that perhaps something was eating people without a trace and this led me to the conclusion that there was a shark loose on the beaches of Santos.
I still can not confirm this because no shark has been found but this is the most succinct and rational theory.

Answer 1

of several bathers at the beach in Santos - at pois é estar em um local, in pois a praia se encontra em santos e não de santos.
finally came to a conclusion - Não precisa do "I"
There were a lot of rumors about - Sem o "talking" rumors já traz o significado de falar sobre algo.
waves that were causing people to drown at sea - As ondas são uma causa para que as pessoas se afoguem, elas não afogam as pessoas.
but when I talked with the lifeguards - aqui talked pode estar certo, mas é um uso informal, dependendo da situação o uso de spoke pode ser melhor, por ser mais formal.
the conclusion that there was a shark loose at sea near the beaches of santos. -No meu ver assim fica melhor explicado por on the beaches poderia ser na areia e ele está no mar.

Espero ter ajudado, apesar de ser fluente em ingles posso ter deixado algum erro passar ou mesmo corrigido algo que não era necessario, mas sei que do modo que coloquei estará correto mesmo se não consegui explicar todas as correções claramente.