Add-on to Sidney's reference seen here: https://www.furaffinity.net/view/36381954/

As a half mernag he is able to transform between merman and naga forms at will. But due to being half, he still retains more of his normal aquatic features and becomes a sea snake.
Sidney initially does not like being in this form. It takes a lot of convincing by Lynn for him to start using it.
He warms up to the land form over time mainly to be with his boyfriend, but he will never admit the comfort of his land form.