"Down hatch" is a colloquial expression that denotes the act of swallowing something. It could refer to eating or drinking something in one go. There are a few synonymous phrases that could be used instead of "down hatch." For instance, "swallow whole," "gulp down," "ingest," and "consume" could be considered as synonyms. Another more idiomatic phrase that could be used is "bottoms up," which denotes the act of finishing a drink. Additionally, "chow down" and "wolfing down" could be used to convey the idea of eating quickly or voraciously. It is worth noting that some of these phrases could have subtle differences in meaning, though they convey the overarching idea of swallowing something.