LIssie, WhatsApp is not very popular in the US the way it is in the rest of the world. I don't know anyone who uses it here, personally, but I'm sure some do. I signed up for WhatsApp but have never actually used it.
epkeytravel, if you and your husband both have WhatsApp installed and can use it to communicate now, then it will work the same in Europe without your Verizon service. Both of you will still need either WiFi or mobile data from an eSIM for WhatsApp to work, though.
If you both set up the WIFI calling feature on your phones, you can selectively turn off the eSIM and then (with phone in airplane mode) turn on WiFI and be able to send/receive texts with your Verizon number (plus make calls) without triggering the Verizon roaming cost. Try it at home that way. Try to call each other using your phone's regular phone app while in airplane mode. This won't really work for communicating with each other (use WhatsApp for that), but it will allow you to check your regular text messages from home once in a while. You might even contact Veizon them to disable international roaming in case you turn it on by accident.