What water saving amendments can I add to soil in my raised planters?

Gel from Plant Centre or Silica Gel or Disposable baby nappies unused ofcourse! as they hold on to liquid

Yes, you can plant marigolds in both planters. Be sure not to plant any other vining plant with the cukes such as squash, zucchini, watermelon - they are not compatible and the vines will take over.

Hello. Here is a link with things you can add.

https://www.growveg.com/guides/3-ways-to-minimise-soil-moisture-loss/

use watering crystals mixed in with the soil for moisture retention

Vermiculite added to you soil will sop up moisture and release it into the soil later. You can do an online search for other moisture-gathering additives. Marigolds can be planted in both planters. You may want to put herbs such as dill, chives, basil or mint with the cucumbersl

Here are a couple companion planting guides. I live in a hot hot hot climate and get a special mix from my local nursery.

https://www.motherearthnews.com/organic-gardening/companion-planting-guide-zmaz81mjzraw

https://www.gardentowerproject.com/news/gardening-ideas-recommendations/why-how-to-companion-plant

vermiculite. It holds a massive amount of water and will release it slowly. Also work in compost. If you are replacing the soil ( a good thing to do every few years for planters) Mix a good quality commercial potting soil with an equal amount of vermiculite for 1/2 of the depth of soil you will have and then 3/4 potting soil and 1/4 vermiculite for the rest. Top dress with finished compost.

Add beans and yellow squash, and maybe a different type tomato to your cucumbers, plus marigolds. There are dozens of types of beans, and you can choose a bush and one or two climbers for poles, tomato cages, or a trellis.

All of these vegetables require watering about twice a week. Before you ever put the soil in, you can line the containers with plastic garbage bags with several pencil-sized holes to allow drainage, but cover the bags with sponge materials, like the soft sheet foam you can buy to make cushions or a mattress topper. Old is fine. This will keep moisture in the bottom of the containers, which is good for the roots and uses the moisture instead of plants’ needing you to water them as often. But drainage helps prevent drowning of the plants. Best wishes ☺️

I'd personally recommend bell peppers or lettuce.

polymer grains-they absorb water,and slowly release it-generally sold as water crystals