Not at all astonishing
Srimad-Bhagavatam
SB Canto 2
The primeval Lord, being all-powerful, can do whatever He likes, and therefore His assuming the incarnation of a tortoise or a fish for serving a particular purpose is not at all astonishing.
SB 2.7.13, Purport: The primeval Lord, being all-powerful, can do whatever He likes, and therefore His assuming the incarnation of a tortoise or a fish for serving a particular purpose is not at all astonishing. Therefore we should not have any hesitation whatsoever in accepting the statements of the authentic scriptures like Śrīmad-Bhāgavatam.
The gigantic work of churning the milk ocean by the combined effort of the demigods and the demons required a gigantic resting ground or pivot for the gigantic Mandara Hill. Thus to help the attempt of the demigods the primeval Lord assumed the incarnation of a gigantic tortoise, swimming in the ocean of milk. At the same time, the mountain scratched His backbone as He was partially sleeping and thus relieved His itching sensation.
The gigantic work of churning the milk ocean by the combined effort of the demigods and the demons required a gigantic resting ground or pivot for the gigantic Mandara Hill. Thus to help the attempt of the demigods the primeval Lord assumed the incarnation of a gigantic tortoise, swimming in the ocean of milk. At the same time, the mountain scratched His backbone as He was partially sleeping and thus relieved His itching sensation.