You have a couple questions to consider. Are you sensitive to the sun? Do you want protection from bugs? If the answer to either is yes, consider ExOfficio.

If not, wear what you have.

If you are sun sensitive and/or have had bad skin problems from the sun, consider www.coolibar.com for sun protective clothing and www.sunprecautions.com for the same.

A cotton t-shirt gives you almost no protection from the sun.