Can I write: "There is 300 tonnes of water"?
We can definitely write: "There are 300 bottles of water" but I am not sure if this is the case with tonnes, dollars, pounds .etc?
I must differ with the other answerer. I believe a singular verb is idiomatically much more appropriate here.
I will quote from Quirk's Comprehensive Grammar of the English Language (1985 edition, pp. 757, 758).
The principle of notional concord accounts for the common use of a singular with subjects that are plural noun phrases of quantity or measure. The entity expressed by the noun phrase is viewed as a single unit:
- Ten dollars is all I have left. ['That amount is ...']
- Fifteen years represents a long period of his life. ['That period is ...']
- Two miles is as far as they can walk. ['That distance is ...']
- Two thirds of the area is under water. ['That area is ...']
Cf: Sixty people means a huge party. ['That number of people means ...']
Please also note, in contrast with the other answer, that ton and tonne are not synonyms. The latter word means "metric ton."
Because you want to say about "tons" (plural) not about "one ton" (single), you should use "are":
(Maybe best way to use the word "tons" instead "tonnes" because this is more frequent form)
Below you can see result of ngram search for "tons", "tones" and "tonnes":