I'm not sure if my post would be helpful or not. But I'll let you know what we do. It's difficult to suggest an amount since every tour is different...some guides are very interactive while others just drive and take you from one place to another with some commentary.
For us, it really depends on whether we received an immersive/interactive experience or not. A good example of the former would be when we took a jeep tour in Cozumel. Our guides were very personable, entertaining and were still able to provide interesting cultural insights. At every stop they had provide a personal touch and allowed us to have time on our own to explore. These guides were experienced in what they did and what they knew. And most of all, they always made sure we were safe and comfortable in everything we did. Other than the extra touches like bottled water, bandanas to cover our faces during the dusty part of the journey, they made the tour fun for every age. With this type of tour, we did provide a good gratuity. But there were no expectations of gratuity as some did not. The guides showed no negative reaction when all they received was a high five.
On the other side of this is an experience we had on a tour during our British isles cruise - transportation and self guided tour in Edinburgh, Scotland. We had a wonderful time exploring on our own which was what we were looking for. However, the guides/driver were there to provide some commentary and personal reflection during our drive, but that was it. So, although we did tip the driver and guide, it was not a substantial amount and was meant to let them know we appreciated their help. Again some didn't tip nor did the guides appear to expect it. We were, likely, one of the first ones to tip since our guide was very surprised when my husband handed it to her...she was very gracious.
For us, tipping is based on the service and not an expectation. It depends on the tour and the experience we receive and generally the way the guides conducted the tour - were they personable? did they ensure our safety and comfort? did they add some interesting facts and made it a fun day?...all these play a part on whether we tip and the amount. As some may suggest a percent of the tour cost, I only take this into account when it's a private tour. With
DCL tours, I believe the cost is much more than it should be and base it on other tours we've done.
The fact that you are inquiring shows you want to do the considerate thing and I'm sure you will.
Have fun on your Alaskan cruise! It's absolutely one of our favourite cruises. We are going again next year.