Sometimes, and it depends on the ship and the slot. Kayakers and paddlers go out during an excursion window, so on some ships you're swapping a landing for the activity rather than adding it to your day. The rule that makes this workable is that activity participants don't count toward the 100 people ashore, which frees up capacity for everyone else.
Worth asking directly about your ship before you book an activity, because operators handle it differently and the answer changes whether the activity is a bonus or a substitution.
Practically, most people who kayak don't regret it even when it costs them a landing. You're on the water among ice for a couple of hours with a small group, which is a different experience from walking a marked route past a colony.
Activities also sell out, often well before departure. If one of them is the reason you're going, book it with the cabin rather than deciding later.