It looks like a sestertius of Roman emperor Antoninus Pius. The reverse shows Roma seated left on a curule chair; behind her, Victory is flying left. Here's an example from CNG's archives: 290, Lot: 395. Estimate $100. Sold for $70. This amount does not include the buyer’s fee. Antoninus Pius. AD 138-161. Æ Sestertius (33mm, 26.36 g, 7h). Rome mint. Struck AD 152. Laureate head right / Pius seated left on curule chair, holding globe, being crowned by Victory flying left. RIC III 889; Banti 455. Fine, dark grayish-green patina, areas of roughness, smoothing in fields. The OP coin does look more like toned silver than bronze. Also, how would a bronze coin survive on a beach? Seems like the salt water would turn it to dust in short order. I didn't dig through RIC-- I just went to CNG's archives and typed "Antoninus Pius, curule". Maybe there are other denominations with this iconography.
Even if it is silver, it will be damaged by salt water pretty quickly and remember that by the middle of the Second Century any Roman denarius would have been considerably debased with copper and that would have accelerated the corrosion process. I do not think this coin has been in salt water for any great length of time.