Both "due to" and "because of" modify nouns or noun phrases, not verbs. For example, it is correct to say, "I fell due to/because of the slippery floor." It is incorrect to say, "I fell due to/because of slipped." In answer to your question, "sank" is a verb, the Simple Past tense of the verb "to sink." But your sentences are still correct because "sank" doesn't follow"due to" or "because of." "the impact" is what follows; this is a noun, so you can use it with either of them.

What rules? Both are fine; sank is a verb, the past simple tense of sink.