Float and amphibious planes are an odd duck (if you'll pardon the metaphor) of aviation because they are one of the only cases where an airplane can change category/class, sometimes in flight. In particular, many float planes spend one season on wheels and another season on floats.
I'll defer to you, the pilot, for how to log a flight in an amphib that begins on water and ends on land, or vice versa. But surely sometimes the flight is ASES/AMES and sometimes ASEL/AMEL
There are a couple of ways to handle this on MyFlightbook.
For the case of an aircraft that is seasonal, I'd recommend cloning it. You can generally do this yourself by simply editing the aircraft's model to be the alternative model. E.g., from a C-172 P to a C-172 (Float); the system will recognize the change and create a new version of the plane. You will then have the option to add the other version of the plane to your logbook, and both versions will then be available to you. You may further choose to activate one version for the summer and deactivate the other, and reverse that in the winter, to make it easier to select the correct version for a given flight. Contact me if you need help doing this with an aircraft.
If the aircraft is primarily on land or primarily on water, then I generally recommend entering it into the system according to its primary mode. Then, if you have a flight that you want to record in the exceptional category/class, you can override it on a flight-by-flight basis. E.g., if most of your flights are entirely on water, then you'd enter the aircraft as a seaplane, and then override the category class to be ASEL/AMEL for the occassional tarmac-to-tarmac flight.
You need to do this on the website. Beneath where you select the aircraft, click on "Show Alternate Category/Class":
This will reveal a drop-down from which you can choose the category/class you want for this flight: