Will photos taken on a cruise ship be blurred?

20 knots per hour shouldn't be a problem. Afterall, the objects you are taking pictures of will be in the distance. I have often taken pictures while on a boat and never had any problems. At least at daytime it works pretty well. If you want to take pictures at night that might be a problem but in that case the tripod wouldn't help much either because of the movement of the boat and the waves.