Yes, definitely a Mexico half-real... The first two pics are of the cross side; shrunk down (see below), you can clearly see the lower L lion, much of the cross, and bits of the upper R lion and lower R castle. That lion style (walking zombie stick-figure) places it right around the early part of Philip V, from about 1701-14. It's remotely possible that this could be late Charles II (no earlier), but every lion style I've seen on a C. II half real is some variant of horizontal-situated rat-cat; have never seen the style of your piece NOT on Philip V. The planchet is the common "teardrop" shape of that late 1600's-early 1700's period.
Can't make anything out of the monogram side from that photo (the last pic is of that side)... there isn't much left there. Perhaps a clearer shot might allow you to see the vague outline of the monogram, which again, should be be Philip's PVS, with a small chance of it being Charles'.
Given the sea-corrosion and the date range, I'd assume this is from the 1715 Fleet.
