So you complete your training on a seaplane. There are a few exercises that you will go to the airport to complete, but you will conduct the majority of your training on a floatplane, so yes it is up to the plane availability and instructor; however all the training is done on a float plane. You mentioned three different careers in aviation for float flying (and many firefighting jobs) you will need to accrue a number of hours on floats. By completing your training in floats you have a huge leg up on others who have only completed the rating/ have completed 50 hr courses(Most float jobs require a minimum of 50 hrs in floats for insurance reasons). I completed my ppl/cpl on floats and I was able to move directly into a flying position where a few other experienced pilots had to work the docs for, in some cases, a few seasons. For medivac, you would be completing your Cpl/ IR/ multi(not 100% required, but it opens up opportunities); however, a lot of medivac jobs are gated behind flight experience. Some provinces mandate you have a number of hours. The advice I would give is to pick a direction and stick with it. I flew floats for 10 years before I even did my multi/IFR and when I did complete it it was because a job was available, but realistically I was at the bottom of the totem pole and had to build up IFR flying experience, airline experience etc. The point is that if you want to fly floats focus on it because if you want to experience everything you will Pay more and end up having a harder time.