Years ago, dive tables were the only option. These days, nearly all scuba divers use a personal dive computer and they should.
A dive computer tracks your depth, bottom time, speed of ascent, and no-decompression limits in real-time. Tables give you a static plan. When you change depth mid-dive, it updates. Tables are set before you get in.
Wrist-mount computers are what most people use at this point. They're small enough, easy to read, and you can use them as a regular watch as well. Console-mount models are an option but not as many buyers pick them anymore.
Budget computers run about $250-400 and do everything the average diver would need. Features include depth tracking, bottom time, no-deco limits, log function, and usually a simple freediving mode. Stepping up to mid-range includes wireless air monitoring, improved screens, and additional full report mix modes.
What people forget is how the computer handles. Certain models are more conservative than others. A cautious algorithm gives you less NDL. More aggressive ones allow longer time but at reduced margin. Both work. It just personal preference and your diving background.
Talk to someone at a dive shop who uses a few different models first. Staff will give you a straight answer on what's good and what isn't hype. The better Cairns dive stores put out product guides and honest reviews on their websites as well