The definite article is there to emphasize which water they mean. The robbers/kidnappers want to make it clear they mean the water of the ocean, not just any water. Otherwise the kidnapped man could immediately spit on his toes, then take off his blindfold and recognize his kidnappers :)
Even though they leave "which water" unspoken, the implication is clear to the kidnapped man because they are on the beach.
Realistically, the first "the" could have been left off and it still would have been clear. The second "the" I would keep, because it poses a question: which water? Answer: The water on my toes, the water that ended my ordeal. It's more poetic this way :)
Your first guess:
Water the first two plants with water and the rest with sirups.
is perfect (except you need "the" before "rest").
As a verb, "to water plants" can stretch to refer to substances other than water.
Another example of the same thing is that you might say "Let's go out for coffee" when you have no intention of drinking coffee. Maybe you'll get tea, maybe some weird new drink they're selling, or maybe even just a cup of water along with a sandwich. "Coffee" is good enough to suggest the location: probably a coffeehouse.
ODO, American Heritage, Macmillan, Collins, and Wiktionary don't list the possibility of using "water" as a verb for substances other than water. This is because dictionaries can't give much idea of how far is reasonable to stretch a word from its primary meaning. One important reason why "to water plants" can stretch so easily to cover other substances is because there isn't any competing verb with a closer meaning.
Here are a few examples from gardening books.
On the other hand, this sounds weird:
Water the plants with water.
Since the primary meaning of "to water plants" is to give them water, this sounds redundant in a way that suggests that there might be a misunderstanding. For this sentence to make sense, there needs to be some context that establishes that watering the plants with something other than water is an idea up for discussion. The first sentence has that: it distinguishes between watering with water and watering with syrup.
By the way, I wrote "syrup" because it's more standard in American English (my native language), so I feel more comfortable with that spelling, but "sirup" is well represented in dictionaries, too.
Best Answer
The Oxford Dictionary says:
In the proportion of 1:1, though not used commonly, is correct grammatically. You can also say in equal proportions, in the 1:1 proportion, or in the proportion 1:1.
I think in the 1:1 proportion and in equal proportions sound more idiomatic.