Can't get the back plate of a Timex Indiglo WR30M watch back on

It is either a screw on case back or a "snap-on. I suspect that it is the latter since you don't mention that you used a tool to unscrew the case back. So getting the back on sometimes is very difficult. I have a special tool for that to apply even pressue and the necessary force and does not put pressure on the crystal. You have to be careful to not damage the crystal. If you don't get it done, then suggest you take it to a shop that has the right tool. If you have very strong thumbs you might get it done.

To open watch case, hold back of watch up with arrow pointer on the back plate pointing to the right. There is a pry point on the upper edge where you will see a small arrow pointing. Use a small flat end screw driver to pry the watch case open. Be sure to replace the back the same way with the arrow pointing away from the setting knob.
Take a 1-1/4 inch paddle bit and drill a hole in a 1-3/4 inch wide piece of wood about 1/8 inch deep. Then take another piece of wood about 2 inches long by 3/4 inch thick by 1 inch wide to lay across the back of the case plate. Place the crystal of the watch in the drilled hole. Used a vise or clamp with the watch between the two blocks of wood to press the back on. Be careful not to over press. Works great

I purchased this watch a few years ago, brand new, the battery died or was not working from the first day. I tried to open it, very difficult, got it opened to see the battery I needed. I could not close the back cover, struggled for a long time, eventually I ended up braking the crystal.
Today 2018-Oct-13 I purchased a battery of another Indigo that needed another battery.
There is a YouTube video, but I still could not get the cover on. So I used a C-Clamp the way he used a plyers. I had to take off the wrist band. I placed the watch face down with a large washer bigger than the crystal. It slip on the washer so I just flipped over my table mat on and it had a nice foam so it did not slip. I used the C-Clamp and still could not get it on. The o-ring seemed to have gotten out of shape and was squeezing out from the sides. Eventually after using a chisel to push the other side of the back that kept popping out when using the C-Clamp I got the cover closed, but the o-ring was partly out, I left it like that.

Quick TIP on How to open and close snap off watch back cover without special tools
Quick TIP on How to open and close snap off watch back cover without...

I tried everything and force wouldn't do it so I dropped by the watch repair place and the gentleman was kind enough to do it for me with the special tool he had and it worked. He didn't charge us which was a bonus.

Thanks for replying.

I found an alternative to the pill bottle cap. The back of my watch is just slightly smaller than a Kennedy half dollar . As I have serious balance problems, I wanted an approach that didn't involve me falling and breaking something.

I put the back into place with the Kennedy half on it, put padding on both sides of the watch. Gripped the whole thing with Vise-Grip® pliers and squeezed slowly until I heard the "pop." Huzzah!

Oh ... fun fact that most magicians know -- a 1901-1907 English Penny is the same diameter as the Kennedy half.

This is probably a bit more risky but I don't have a vice or the wherewithal to make recesses in bits of wood. Taking the back off is reasonably straightforward with a small screwdriver positioned alongside the arrow as others have indicated. After replacing the battery I snapped the back on by lining up the back in the required position and placing a UK 1p coin on the front and back of the watch and using a long handled pair of pincers to apply pressure. It worked for me - although I imagine too much pressure might have broken the glass. Good luck.

Direction of arrow on back is not important except perhaps to help you open the back. It opens easily enough if you drop it on the floor by mistake, incidentally
To close it just take two pieces of wood not wider than 1 inch for the back and any size for the front where the crystal resides. Pop the whole works in a vise and with little pressure, the assembly pops together.