Does she love going out on the boat? If so, maybe respond with "I know how much you love boating, have fun"...
when you mention what your doing, your not mentioning validating, you need to add that, it's one of the most important things for now.
Also, regarding her critiquing your parenting, have you cared for your son in the past? If not, and your really having difficulty figuring all out, you may want to have a conversation about it. I would start by saying that you are not ok with her constant critiquing of the care you are providing son, but you understand she is concerned about different schedules impacting your S, so you would like to discuss how the care can be provided by both of you seemlessly.
Create a schedule / plan, include nap times, meal times, play times, etc., anything that needs to be addressed.
I think this will allow you to let her know you are not ok with being critiqued (boundary), validate her feelings, and show that you are a great dad and want the best for you S.
I am not a vet, but these are my thoughts on your sitch.
M - 9 1/2 years 5/5/16 - Bomb drop - 3 week EA 10/31/16 - We sold house 01/10/18 - D Finalized