Thanks for your question! I saw your same question in my email and also wrote back via email - but here is my full response here as well, in case it's helpful to others! I also added just a few additional clarifying sentences to this response that occurred to me, versus the email I sent.
Here is a good page on the site to look over for swim watches: http://www.bestfitnesstrackerreviews.com/activity-trackers-for-swimming.html
In particular, the watches listed on that page that will have the best swim metrics are:
Tom Tom Multi Sport and Multi Sport Cardio
Garmin Swim Watch
Garmin Forerunner Watch (910 and 310)
Garmin fenix2 watch
None of the above watches are able to measure the heart rate from the watch/from the wrist while you're swimming. There are three products that can measure heart rate while swimming:
AquaPulse Heart Rate Monitor
Instabeat Swim Monitor
Poolmate HR Watch + Under Water HR Chest Strap
Fitness trackers that can measure heart rate (while exercising, not just resting heart rate) from the wrist while on dry land include the following. However, these do not track swim metrics:
FitBit Charge HR (iOS, Android). This is a good all-around fitness tracker.
FitBit Surge (iOS, Android)
Mio Alpha (not sure if both iOS and Android) - this only measures heart rate
Basis Peak Watch (iOS, Android)
As for which ones sync to both iOS and Android, I've filled in the ones I know for sure. But it will also depend on whether your iOS or Android device is compatible with the specific list on each company's site -- usually, it has to be a recent enough device to have BLE connectivity. For this reason, it's best to have your device types written down, including the software versions, and call the company to make certain before hitting the "buy" button. Most also have a list online of compatible devices, but these lists can take a little clicking around to find.
None of the fitness trackers will automatically track cardio exercises like pilates, but if they track heart rate they should be able to accurately estimate calorie burn. The FitBit products and Basis Peak both also track running/walking metrics (steps, distance, calories, etc).
If anyone else has helpful advice or can point to some products I forgot to mention, please chime in!
Hope this helps!