副词修饰动词thouands of lanterns slowly drift out to sea.the boat

两句都对：
1. 副词修饰动词,这点应该是没什么问题.
2. 至于副词的位置：副词修饰动词,其位置很活泼,一般可以放在动词前面或者后面：
比如：
She danced beautifully.=She beautifully danced.
3. 另外：含有介词的副词修饰动词,其位置：
动词+介词+宾词+副词=副词+动词+介词+宾语=动词+副词+介词+宾语:
所以：Thousands of lanterns slowly drift out to sea. =Thousand of lanterns drift out slowly to sea. =Thousands of lanterns drift slowly out to sea.
所以两句都是对的!
注：有些副词位置改变句意会改变：
You know well that the girl can’t drive.
You know that the girl can’t drive well.