I've read that there may be small opportunity to catch certain species on their journey to/from warmer waters of Mexico and even Hawaii during the later months of the year or earlier months of mid-spring, April for example.
However, the recognized whale watching season for Alaska is May through September. If you're lucky, you may see some from your cruise ship but you can certainly make an excursion of the event as there are plenty of pre-planned activities offered within the cruise industry or as result of the presence of cruise passengers.
You might catch glimpse of the gray whale, beluga, orca (killer whale), regionally renown humpback whale, and even the gigantic blue whale!
Here's where Kenai is, a little west from Anchorage and somewhat northwest along Alaska's Pacific Coast from where many glacier-viewing cruise itineraries stop in Juneau.