Outside in the weather, or outside under a shelter of some kind? What is the weather like in that part of the world? The method used to make your table may largely depend on how much water, direct sun, and freezing it will typically endure.

Is under shelter and warm climate, no snow, direct sunlight. So direct weather damage wont be a problem.

I think you can do the table as you envision. Make sure the boards are very dry: The stock from the home improvement store will still be quite wet. Work with 8% moisture content or less. Glue them up edgewise using a good water-resistant adhesive, such as Titebond II. Leaving at least 15 cm at each end, cut the hole for your tub in the center. Seal the entire thing with a marine tung-oil preparation (e.g. Waterlox) to protect it from mosture, and see how it works.

Love the idea ! Hope your DIY skills are good ! If not, buy a preloved dining table from ebay and cut out a trough to fit a aluminium feeding trough maybe ?!

I would love to see pictures as your project progresses :-)