I agree with the others that saying you'll attend Memorial in Puerta Vallarta while in port is the way to go...Iits seen as kind of cool to celebrate with our bros and sis in other countries...and actually they would LOVE to have you join them!!
I've been to the KH there and its actually quite nice!! If you have the chance and want to carry the charade a little further you could go by and take a picture of the KH and send to your mom!!
bythesea (btw, Welcome to the board!!!)