If the Torah says all animals that have scales have fins how does a sea snake have scales and no fins

The term scales according to the Torah refers to visible scales which can be peeled using the nail (like a shell or a peel). The sea snake and similarly the swordfish have microscopic scales which cannot be peeled off using the nail. Therefore, according to Torah, they are not considered scales. (Rema Yoreh Deah 83, 1. Maor Hakashrut Rabbi Levinger P. 308. Hamahapach Rabbi Zamir Cohen vol. 4).