The main advantages of the waterproof "Advanced"
Marco Polo vs the "Ultralight" are; IP67 waterproofing, clip-in mounting system that allows you to easily remove it for charging or to move to another drone and a crush-resistant polycarbonate case that will protect the tracker in the event of a direct impact in a crash. The Ultralight is smaller and lighter and fits on drones as small as a Spark. Both have the same range and battery life. There are some great examples of mounting both types of tags on this forum, just search for
Marco Polo Mounting.
Regarding the need for waterproofing, if you were flying a Matrice 200, which has an ingress protection rating of 43 and a payload capacity of over 5 lbs., you definitely wouldn't want to save the $15 or the 0.6 oz. of additional weight and NOT get the Advanced tracking tag! For the Mavic Pro, it is not quite so clear. The most frequent story I have heard is guys having a fly-away late in the day and having to come back to search after it rained overnight. If the tracking tag is not functional then you are much less likely to find your rig, even though it may be in pretty good shape after a light rain.
Regarding transmitting from under the water, it is not quite as bad as you may think. We have done testing and found that, in freshwater, there is about 24dB of signal loss for each 6 inches of submersion. What this computes to in terms of range is that 6" of submersion will reduce the open field range from a maximum of 2 miles to about 1/3 mile, at 18 inches of submersion you may get about 400 feet of range and the depth at which you would have to be standing right over the drone to get a signal is about 4 feet. So, underwater recovery is possible - if it is not too deep. What salvage value to drone has is another story but, in some cases, just getting the SD card back may be worth getting wet.