I did the velcro for a while, but it gets cumbersome. You have to position it very carefully to avoid obscuring the down-sensors on the Mavic Pro. Also, it can interfere with folding/ unfolding. A rubber band like from a bundle of asparagus seems to work. (Don't buy broccoli. Broccoli does not come with a rubber band and you have to eat the broccoli.)
With my drone, a Mavic pro, I attach the battery by carefully pressing down first on one side, then the other. You should get two distinct clicks. If you just press it down in the normal manner, the clicks tend to sound on top of each other, and you don't know if both sides are latched or only one. This method has given me security over hundreds of flights.
Next: The rice trick can work. My drone got wet one day from rain and flew home OK, but would no longer boot. I bought a fresh box of rice and put about 2-3 pounds of it in a plastic bag. I stretched a pair of socks over the folded drone / battery to keep rice dust away from the innards, camera gimbal, etc. Placed the affair in the sun on mild days. Be careful of hot days, it might get hotter than you want, and I don't know what the limit is. On the second day I began to see moisture condensing on the inside of the bag. Eventually the rice gobbled it up, and about three days later I was seeing no more condensation. Left it out in the (mild) sun by itself (no bags or rice) for another day and tried it. Booted normally and trouble free ever since. Good luck. If salt water, I would not expect success.
