You might define what you mean entirely by “work with.” Whether you just mean, can you control your HA’s via having a version of your HA smartphone app on the watch, too, or whether you also mean, “can I stream from the watch to my HA’s?”
As far as working to control your HA’s, when you look in the iOS App Store, a little bit of script under each app will tell you the devices on which the app can work. So, if the script doesn’t say “Apple Watch,” you won’t be able to add a version of the HA app to your Apple Watch from your iPhone.
As far as streaming goes, AFAIK, Apple Watches don’t do MFi connectivity. They do stream classic BT and as such you can stream them to devices that connect via classic BT, for example, regular old earbuds or Apple AirPods (no firsthand experience here myself). Or, if you have an intermediate streaming device for your MFi HA’s like a Phone Clip+ for ReSound, you could stream to your HA’s by streaming through the intermediate device with all the latency problems, etc. I should think with the current line of Phonak devices, you could stream directly to your HA’s via classic BT (Marvels, Paradise, Lumity, etc).
IIRC, though, the thing about the Apple Watch 8 (including the Ultra) is that these devices are BT 5.3 capable. So, whenever, if ever, Apple decides to come out with support for BT LE Audio, one could presumably stream directly to any HA that is BT LE Audio capable. The Apple Watch Series 7 only supports BT 5.0, according to Apple’s specification listing: Apple Watch Series 7 - Technical Specifications.
I have ReSound Quattros and the ReSound Smart 3D app works great on my Apple Watch Ultra. But one has to have connectivity to one’s iPhone, presumably because it’s the iPhone that’s doing the “talking” to one’s HA’s via its MFi connectivity. Ultra watch control of the HA’s works with either BT or Wi-Fi connectivity to my iPhone, IIRC. Haven’t tried the watch HA app connectivity to my iPhone via cellular yet. According to Apple, the watch intelligently prefers BT to Wi-Fi to cellular because of the increasing battery drain of those types of connectivity. So, that’s awesome, IMHO.
BTW, as far as cellular connectivity goes, anyone who has Spectrum (Charter) Internet cable service should look into switching to Spectrum Mobile just for Apple Watch cellular connectivity. I was an AT&T Prepaid (AutoPay) customer previous to getting my Ultra. I paid ~$44/mo for 6 Gb data c unused 1 mo rollover, unlimited calling & texting. But Verizon & AT&T only support the Apple Watch on postpaid unlimited accounts, prospectively costing me >$90/mo or >$80/mo total, respectively, to include my Ultra on the unlimited cellular plan I would have to carry.
With Spectrum Mobile (if you have Spectrum Internet), I’m on a By the Gb plan @ $14/mo/Gb. I pay $10/mo for the watch, with no added taxes or surcharges for either part of the plan. Since I’m mostly at home, my calling and surfing is almost via our Spectrum Internet cable service (~$80/mo) by Wi-Fi. The Spectrum cellular service is MVNO and provided by Verizon, so it’s quite good and includes 5G connectivity (the Ultra is only 4G LTE-capable, but I use the 5G, where available, with my iPhone). I get international roaming in Canada, I think it was $.01/call, $.02/text, and $.01/Mb data. So, for my low outside-my-home data usage, I’m paying $24/mo (in addition to an $80/mo cable charge that I’d pay anyway) for unlimited calling and texting and Apple Watch Ultra cellular connectivity. Based on my former ~$44/mo AT&T Prepaid payment, I will effectively pay for my Apple Watch Ultra in a little over three and a half years by the savings on switching to Spectrum Mobile.
One final note on Spectrum Mobile. I bought my Ultra as an unlocked device, directly from Apple. It took something like Tech Level 3 Spectrum Mobile Support to get it connected to the Spectrum Mobile network (Verizon) and a week or more of waiting. It seems like an Apple Watch Ultra sold by Spectrum Mobile works great out of the box for cellular connectivity. Since my Ultra was one of the first delivered by Apple, hopefully Spectrum has improved the out-of-the-box cellular connectivity for unlocked Ultras since then.
So, I’m also in the market for new HA’s. A major consideration is how they help me hear speech-in-noise. A modest consideration but important consideration is how they work with my Ultra. If I were picking new HA’s solely on how they worked with my watch, I’d go with ReSound and its fantastic Smart 3D app (and the great HA rechargeable battery life with the option to use disposables if one prefers those). However, in the ReSound Omnia vs. the Phonak Lumity thread on the forum, several experienced ReSound users prefer the Lumity over the Omnia for sound quality and speech-in-noise ability, even though they are only available for now as rechargeables with far inferior battery life compared to ReSound. I don’t believe the My Phonak app runs on an Apple Watch.
So, I will definitely be checking out the Lumity and if I have the same comparative experience that JordanK and Ureout have had vs. the Omnia, foregoing any use of the Ultra to control my HA’s. There are a ton of other great features of the Ultra that, IMHO, make it well worth its expense. Apple can change the battery for $99, too. Don’t know what the risk is with that to its waterproofness, etc. But the ability to get a new battery made it much easier to swallow the price and hopefully Apple will support the watch for many years to come and even more wishful thinking on my part, support BT LE Audio streaming from the Ultra someday to all BT LE Audio-compatible HA’s and other devices.
Edit_Update: I see that I already made a similar post in another thread: Resound Omnia vs Phonak Lumity - #33 by jim_lewis. Sorry about that. Memory is definitely decreasing with age and I should have checked more thoroughly before posting in somewhat similar detail here.