You need to put the checking thing in a permanent timeout loop.
Try this, not sure.
NPC Code:
//#CLIENTSIDE
if (playerenters || timeout) {
if (onwater(playerx+1.5,playery+2)) this.wcount += 0.05;
else this.wcount = 0;
showimg 1000,@@c@Water Count #v(this.wcount)/15,screenwidth-75,screenheight-25;
if (this.wcount>15) playerhearts = 0;
timeout = 0.05;
}