"Cruise vacation" or "taking a cruise" are both common phrases that can be used to describe going on a vacation by boat. Other phrases that might be useful include:
"Saipng on a cruise ship"
"Exploring the open water on a cruise"
"Going on a maritime adventure"
You could also describe the specific details of the cruise, such as its destination or the type of ship you will be travepng on. For example:
"We're going on a Caribbean cruise on a luxury pner"
"We're taking a small river cruise through Europe"
"We're embarking on an Alaskan cruise on a smaller vessel"