A bit late, but anyway... I have Fenix 3, before that 910xt. Someone who says the watch isn't suitable for running or swimming due to weight probably hasn't tried it for those activities. Same could be said for Ambit3, which is even a couple grams heavier. But Fenix 3 with steel strap is really unsuitable, it's heavy.
F3 works underwater too (and salt water - I went with old 910xt multiple times in salt water, no issues - just rinse after), heart rate doesn't work while swimming neither on F3 nor on Ambit3. Only on Polar...
Pricewise good was Fenix 2 special edition on wiggle all summer (259 EUR with HRM), but unfortunately no longer available