Cutting tools are a controversial topic. I’ve never heard anyone arguing about them, but when some divers are diving with a 10 inch knife strapped to their leg and others with only a 2 inch plastic line cutter on their wrist, it’s clearly something people have wildly varying opinions on.
Knives
Knives are the traditional option. They’re probably the first scuba accessory that was available. I would recommend going smaller. With steel there is a trade off between hardness and corrosion resistance, so that is why titanium is a popular option. Blunt tips are popular and allow you to use the knife for prying while making an accidental stabbing less likely.
Line Cutters
Line is far and away the thing you will most frequently need to cut underwater.
Abandoned and in-use fishing line is unfortunately all over dive sites.
Deploying a DSMB is very easy to mess up and can easily result in you being tangled in your own line.
Lastly, the best way to attach you gear to yourself is with a bolt snap attached with line, so it can be easily removed if it somehow gets stuck.
Recommended Cutting Tools
Basic Cheap Ceramic line cutter
Glow in the Dark Eezycut with steel blade
Titanium Cutter
Titanium Blunt Knife
Scallop Knife
Shears