I have used [Cetacean Research's SQ26-H1 Portable Underwater Sound Recording System][1] for listening for fish sounds from a boat, similar to what you're proposing with whales, and I believe the system should be at least close to within your budget. I was able to get a student discount for mine, so I can't recall the usual full cost of it.

The paper discussing the relatively new HydroMoth prototype ([Lamont et al. 2022][2]) also presents the results of a systematic review of recent acoustic studies and the costs of the recorders used by their authors, to quantify the lack of low-cost underwater recording equipment, especially when compared to terrestrial recording equipment. So you're not alone in struggling to find affordable options!


  [1]: https://www.cetaceanresearch.com/hydrophone-systems/crt/sq26-h1/index.html
  [2]: https://doi.org/10.1002/rse2.249