Taking bones apart and extracting marrows was something mages did all the time. Generally speaking, bone marrow was something used for making blood, but the bone marrow of many magic beasts was stuff used to repair their magic nuclei. Their bone marrow also contained various special materials. It could serve as excellent magic material when extracted and refined.
The one vital shortcoming of sea magic beasts when compared to those on land was that they did not have as much marrow. The same went with blood, as well. Fortunately, sea magic beasts were commonly of large sizes. A low-level sea magic beast could have a length exceeding ten yards easily.
