“水乳交融”常见的英文表达有 “be in perfect harmony”、“be completely/intimately blended” 或 “be as closely united as water and milk” 。以下为具体解释：
be in perfect harmony： 强调双方关系或状态高度协调、一致，毫无冲突，和“水乳交融”所表达的亲密无间、高度融合的意境相符。例如：The couple's relationship was in perfect harmony, just like water and milk.（这对夫妻的关系水乳交融，就像水和奶一样。）
be completely/intimately blended： “blended”有混合、融合之意，“completely”或“intimately”进一步强调融合的程度之深，能传达出“水乳交融”中那种紧密、不分彼此的感觉。例如：The two cultures have been completely blended, like water and milk.（这两种文化已经水乳交融，就像水和奶一样。）
be as closely united as water and milk： 这是较为直白的翻译，直接将“水乳交融”的字面意思表达出来，让读者能直观理解这个成语所描述的紧密结合状态。例如：The two teams worked together as closely united as water and milk.（这两个团队水乳交融地合作。）