I wonder... it might be worth it trying some (inexpensive) sand stakes from a camping store. The ones that come with the sportbrella don't seem to be as good as something like these or even these.

I've also had luck with sandbags - you just get something like these or these and fill them with a little trowel at the beach, then empty when it's time to go. (Honestly, at the beach, cut-out gallon jugs work just as well... but I'm not sure where you'd get a gallon jug on a cruise!)