Why do dry plants have more stomata than aquatic plants?

You may find an answer in these articles:

The surprising fact that leaves of xerophytes are comparatively manyOpeningshave, is explained by the fact that they can thus ensure maximum material production in the very short time of sufficient water supply: Many gaps allow a high rate of transpiration and a rain gas exchange. (An exception to this is theSucculentwhich have a small number of gap openings. However, the succulents are often not counted to the xerophytes, but are considered independently: unlike succulents, xerophytes do not have pronounced water storage tissues.)

http://www.biologie-lexikon.de/lexikon/xerophyten.php

Some desert plants develop only thorns instead of leaves. In the case of conifers, the shape of the needles itself already reduces the transpiration. Furthermore, a wax-like layer,recessed openingsand a thick cuticula for the smallest possible evaporation.

https://www.biologie-schule.de/trocken Pflanzen-xerophyten.php

To save water in dryness, closePlantsfor example, their gap openings on the underside of the sheet. This evaporates water, but also introduces carbon dioxide necessary for photosynthesis.Plantsalso roll in their leaves in dryness or throw them off immediately.

https://www.quarks.de/umwelt/klimawandel/so-gene-plant-better-mit-heat-und-dry-clear/

https://www.biologie-schule.de/wasser Plant-hydrophyten.php