It will really be dependent on what is your physics field but you can definitely survive in physics without deep knowledge of fluid mechanics except when your study require it
PS: I am a particle physicist.
I chose to take a particle physics course as an elective in my final year - I was planning to specialize in battery and capacitor technologies and wanted to learn more.
The lectures were very different to Engineering much, much more theory focused(almost nothing on applications) it was my introduction to things like Hamiltonians, Wave Functions and Fermi-Dirac statistics. I'm glad I took the course I learnt a heap especially about semi-conductors it gave me a better appreciation and understanding of things we covered in my engineering degree like magnetism and phonons/heat transfer as well. But I will say it did feel like another world compared to Engineering - there was much less in common than I would have thought.