exiting at praça xv, boats can be taken every 30 minutes going to the maritime terminal in niterói - praia grande is only a 5 minute walk away from the ferry terminal. 

on this link, the institudo estadual do ambiente - inea, provides the bathing suitability of several beaches in niterói, including praia de boa viagem, which is close to praia grande and can be used to guess water quality.

constantly updated, the red flags mean that swimming is not advisable, green flags mean waters are clean, and yellow flags suggest using own judgment.