"Companion planting" is a myth. Plants don't generally like or hate each other (well, some allelopathic plants do try to kill other plants). Some plants can repel insects that might harm another plant. Cukes and tomatoes are pretty much irrelevant to each other.

You can plant anything next to each other as long as the plants aren't competing for sunlight, rootspace, water, and nutrients. Oh, as well as making sure they aren't impeding the airflow around each other which could promote diseases.

The only actual evidence of companion planting working I have found is planting onions near carrots to help repel carrot rust fly. And even then, you have to plant 3 rows of onions to one row of carrots to have the repelling effect.

Rodney

Wow, mind blown.

But I have read that planting watermelons next to cucumbers will give you...watery cucumbers or something. Is this not the case?

It is not the case :-) They belong to the same plant family and can be easily and safely grown side by side.......provided you allow plenty of room for both plants.

btw, cucumbers are watery plants by nature! Most sources state 95-96% Watermelon is only 92% water

FWIW, regular cukes and watermelons can't even cross-pollinate. The "cucamelon" fruit, which look like miniature watermelons, are incorrectly thought to be a cross between cukes and watermelons. They are not. Mythology abounds!