Years ago, dive tables were the only option. These days, most divers dive with a wrist-mount computer and for good reason.
Your computer calculates depth, bottom time, speed of ascent, and no-decompression limits in the moment. Dive tables are a fixed calculation. If you go shallower during a dive, it updates. Tables are set before you get in.
Watch-style computers are what most people go for these days. These are compact, readable underwater, and you'll wear them as a watch as well. Hose-mounted computers are available but not as many buyers pick them now.
Entry-level computers go for around a few hundred dollars and webpage cover everything the average diver requires. You get depth, time, no-deco limits, a logbook, and usually an entry-level apnea mode. Mid-range gets you air integration, better screens, and extra gas modes.
What people forget is how the computer handles. Certain algorithms are tighter than others. A cautious setting means reduced no-deco time. Looser ones extend bottom time but at a thinner margin. Neither is wrong. It's your style and how experienced you are.
Talk to someone at a local dive store who's used a few different computers first. They'll offer a straight answer on what works and what isn't just marketing. The better Cairns dive stores publish gear reviews and honest reviews on their sites as well