Question: Can I say Tehilim on the sea side without a shirt on?
Answer: One should dress respectably when praying or saying Tehilim. But there is no Isur (prohibition).
I love this particular question and the response - which is totally appropriate and subjective. The questioner is comfortable enough to daven at a beach or felt moved to pray after bathing topless. The rabbi recognizes that there is a need to promote proper decorum in prayer but that there is no real restriction to spontaneously say psalms.
Think about it - who wouldn't be at a beach, pondering the eternity of the tide and not think to praise Gd and nature's amazing cycle? Just to bridge to a post last week on davening on airplane - I am not sure I would recommend gathering a minyan of people to pray as it might bother the other sun worshipers.