PolarPro makes nice equipment, it's hard to go wrong with them.
They have 3 options,
-single red filter for 15-75 feet for $30
-pack of 3 that includes snorkeling depths and deeper than 75 for $60, but they are the same design as the single filter, so you can just snap on one at a time, not changing during the dive.
-or a system for $100 that includes 2 filters (I think 15-75 and deeper than 75) plus a magnifying glass for close ups, that lets you swing the filter or magnifying glass out of the way. In theory, you could use without any filter for snorkeling and shallow stuff, swing the red filter in for deeper stuff, and swing the magnifying glass in and out as desired. This is probably the most flexible if you are doing a mix of depths and a mix of small fish and larger animals on the same dive, but it's also the most expensive.
On Amazon there are tons of options for $10-$15, and as much as I like the Polar pro design and quality, if you only got one filter from Polarpro but wanted some backups, I don't know if you'll see a huge difference with the cheap filters. For example, I have a small scratch on my gopro lens--it's a 7 so it's easy to swap out, but I haven't felt the need, I don't notice it in videos.
The 8 is a huge, huge, huge jump up from the 3+. I found myself making incredibly more videos since it is so much easier, the quality is higher, transferring to phone to make a Quik video takes just a few minutes, and I have a good time at the end of a day spending 5-10 minutes throwing together a short video of the day, sharing with family.