Many thanks to all who replied to this (mostly off-list). The general consensus seems to be that the word refers to bottle gourd, and I see that some sources, including Wikipedia, use the word calabash. Rajam's comment about the use of these gourds as swimming aids (obliquely confirmed by Aleksandar's Mokṣopāya/Yogavāsiṣṭha reference) was particularly illuminating. I was beginning to wonder whether I had misunderstood the image, as the author doesn't actually say crossing the ocean, but simply diving into it (vigāhe). The swimming aid makes perfect sense.

Here is the verse:

tārtīyakasiddhāntād yathāsvamatyoddhṛtaṃ śāstram |
kaṭutumbīphalavad idaṃ grāhyaṃ pṛcchārṇavavigāhe ||

'Let this teaching, extracted from the system of the Tatars according to my own understanding, be used like a calabash on diving into the ocean of [astrological] queries.'

Best wishes,
Martin Gansten