- The big breakers came in slow.
- The big breakers came in slowly.
Are there any differences between the two sentences?
"Slow" and "slowly" are both adverbs but they are not completely synonymous with one another. A usage note at the entry for the adverb slow in Merriam-Webster's Collegiate Dictionary explains:
Slow is almost always used with verbs that denote movement or action, and it regularly follows the verb it modifies (beans . . . are best cooked long and slow —Louise Prothro). Slowly is used before the verb (a sense of outrage, which slowly changed to shame —Paul Horgan) and with participial adjectives (a slowly dawning awareness . . . of the problem —American Labor). Slowly is used after verbs where slow might also be used (burn slow or slowly) and after verbs where slow would be unidiomatic (the leadership turned slowly toward bombing as a means of striking back —David Halberstam).
No, there is no difference in meaning between your two sentences.
In this sentence most people would say slowly, but there is no difference in meaning.