I don't think the problem is that people would not know what you meant, but that it would be bad grammar in Hebrew. Remember, different languages do things different ways. We are learning the way to say things in Hebrew, which will not always translate word-for-word into the way we express the same thing in English. I see you have been studying Spanish longer. We might wonder WHY do they usually put the adjectives after the nouns in Spanish, or WHY do they change the verb for every "person" (1st person, 2nd person, etc)... it is just the way to do it in that language! :) In Hebrew, if the direct object is definite, you have to put את before it :)
I understand your response, but in fact, I have found that every language has its inherent logic to most (though not all) of its grammatical rules. Although the logic is unique to that language or language family (it does not make intuitive sense to speakers of other languages), one of the breakthroughs in learning a language comes through cracking that language's inherent logic. I suspect that the obligatory use of את before a definite direct object might be such a case--in other words, I am asking if "!סוסים לא שותים זה" actually means something, other than just being bad grammar.
There are some exceptions to את being used as a definite direct object marker. Specifically, before זאת it is often left out, and in poetry. But it all names are definite, all construct chains ending with a name are definite, all construct chains ending with a possessive are definite, all construct chains ending with a definite noun, all possessives... Even small and innocent looking words such as כל can be the start of a construct chain. It is even sometimes needed with question words. It is always there, waiting around the corner to trip you up. :(