The simplest solution is a Speed/Depth/Temperature transducer networked to a Chartplotter, like the Zeus or Vulcan. This will give you all the information you need, an electronic chart, speed, depth, and speed over ground (SOG) and course over ground (COG). It may not give you heading unless you have a separate electronic compass.
A chart plotter is only necessary if you want or need the gps and charting function. If all you want is speed and depth a single display and a single transducer is all you need.
Tablets and phones work too. However, they have 2 important limitations, battery life and sunlight visibility. I have new iPad (M2) and the best I can get is about 2-3 hours of use before the battery is dead. There are ways to get around this limitation with external batteries and USB charging ports. Sunlight visibility will be an issue with most tablets.
The Chartplotters usually have a preinstalled base chart with basic information. It is necessary to purchase separate charts if you want detailed information, that may or may not be necessary. When I sail on Lake Ontario, detailed depth information isn't necessary because the lake is really deep. However, on the ICW and other areas detailed depth information is really important.
Most (all?) transducers are made by Airmar, so that should not sway your decision. A 7" display is small, a 9" display is easier to read, and a 12" display is pretty big and expensive. I find touch screen only devices hard to use, I much prefer knobs and buttons in addition to touchscreen (I have one of each). However, devices with knobs and buttons cost more.
If you ask about brands, there are 3 major brands, Garmin, Raymarine, and B&G (which includes Simrad and Lowrance, same devices oriented to different markets mostly through the software). Pick a brand and you'll find lovers and haters. B&G has the longest history of catering to sailors, over the past few years it has fallen prey to private equity firms and is now owned by Brunswick (along with a bunch of other marine companies and bowling ball brands). The frequent transitions have not had a positive impact on the company. Hopefully, they will find some stability with Brunswick.
Garmin has long been a high quality brand in the Aviation world, not so much in the recreational marine world. However they have been on a tear buying up companies to build their brand, including Active Captain, Vesper Marine, and Navionics. Defender.com would not carry their products until the company was sold to a Canadian company a few years ago.
Raymarine began as a subsidiary of Ratheon Corp, yes the Defense contractor. It was spun off a long time ago and has had its ups and down. It almost went under until Flir bought them. In the past few years the products line seems to have stabilized. One drawback to Raymarine, at least in my eyes, is the use of proprietary connectors for the networking. Garmin and B&G both use standard NEMA 2000 connectors, that makes adding additional devices easier.
That may be more than you wanted to know. Good luck.