If you are going on an Alaska cruise, chances are you are leaving from Vancouver,Canada, or Seattle, Washington. From either of these departure points it is easy to get to Victoria, and certainly worth a visit.
From Seattle to Victoria, hop aboard the Black Ball Ferry Line at the cruise port. From Victoria to Vancouver, use the BC Ferries to get to the Vancouver port. In fact it is possible to fly in to Seattle, never rent a car & get to Victoria, and on to Vancouver using these, and other services. From Seattle Airport there are 3 ways to get to the cruise port, a taxi around $40, a shuttle currently $22 per person, or the metro & a taxi, as the metro drops you off 5 blocks from the pier. The pier and downtown are in the same area.
This article by The Seattle Times is an excellent rundown on the transportation options between Seattle, Vancouver & Victoria.
Important to keep in mind is that Canada has a different currency than the US, and you will need to exchange funds to use in Canada. While some Canadian stores & restaurants will take US dollars, busses and ferries usually do not, so better to be prepared.
The ferry from Seattle to Victoria is a short trip, and during high season there are up to 4 trips a day. Downtown Victoria is a compact, walkable city. Be prepared with a jacket, even during the summer, as it can get chilly in the evening. If you are Traveling on from Victoria on to Vancouver to board a cruise, you can even include a tour of Butchart Gardens with transportation on to Vancouver using Prince of Wales.
A public bus makes the trip to Butchart Gardens and this website tells you everything you need to know about what flowers are blooming, the cost of admission, the dining options available & of course how to get there. Admission is currently $30.20 for adults, and between $3 and $15.10 for children, plus taxes and fees, all in Canadian funds. The amazing thing about the gardens, is how widely different the flowers are from season to season.
No matter how you get there, be sure to schedule some time in to visit this beautiful gardens, it is well worth a visit.