DCL has gratuity information on their website in a table:
https://disneycruise.disney.go.com/faq/onboard-services/gratuities/
I have mobility issues but do not need to use an ECV on cruises (yet; I do need an ECV at WDW; I do not need one at home).
Most DCL port adventures are NOT ECV compatible. This can be for any of a range of reasons as noted by one of the previous posters. More are manual wheelchair accessible, but even those are limited in number.
On the DCL website they have descriptions for each port adventure. On the port adventure's individual page they will have information about whether a given port adventure is wheelchair or ECV accessible and notes about restrictions or limitations. DCL port adventures info:
https://disneycruise.disney.go.com/port-adventures/overview/
Also, many of the port adventures are similar or identical to port excursions offered by other cruise lines [in a given port, the cruise lines will often use the same excursion providers often with the same or very similar set of tours]. When they are similar, often Disney may have a slight variation to "disney-fy" it, the difference may be somewhat minor and you can usually figure it out by reading the descripton. So, if you narrow down to a short list of port adventures you are interested in, you can then do some Googling of them and see if other cruise lines basically offer the same ones and then see if you can find reviews of the equivalents from cruisers on those other lines - it will open up your options for information-gathering.
You may find some useful information at the Cruising With Wheels youtube channel:
https://www.youtube.com/c/Cruisingwithwheels/featured . Per their "about": "
We are a married couple with one of us in a wheelchair. We find cruising to be the best option for our vacations. Our channel focuses on everything about cruising and primarily handicap accessibility. You will find ship accessibility, cabin tours, ship tours, shore excursions, vlogs of our adventures as well as other useful informational videos." I find them useful for general cruising info as well as special needs tips. They often do NOT do cruise line port excursions when in port so you will get some good ideas about options of things to do and how to plan things to do in port from them.
Hope this helps.
SW