K.F.
Depnding on how overweight your husband really is walking may be very uncomfortable for him or he may prefer to walk in silence instead of having conversation and walking at the same time.
Honestly he is a grown man and mothering him or treating him like one of the kids is never going to work on him. If he likes golfing, there is a certain amount of exercise that comes with that. Let him do that one his own and you do your thing on your own. It isn't the end of the world.
Try to find some other activity you guys can do as a family. It may take some thinking outside of the usual box. If he is competitive perhaps something that will ignite his competitive nature will be helpful.
When in the pool encourage the kids to engage their father by you actually spending more time outside of the pool away from them all. This may not be exactly what you had in mind but it may get him more involved. Perhaps borrowing a bike and going biking may be more up his alley.
Try different things and see what sparks interest in him. If nothing works then it may be time for him to see a physician because perhaps his testosterone levels are lower than they should be which may be the root cause of his more sedentary nature. I hope this helps and encourages.