Conflicts between your iPhone and the ship's wifi:
 

On a recent cruise I was having a hard time getting my phone to work both as a phone and as a wifi hotspot for my laptop, even though the phone was showing a cellphone signal. After asking a large number of crew I finally found out that my iPhone was being switched by the ship's stronger wifi signal to the ship's wifi rather than the normal cell network. The iPhone has an option to tell it to ignore any particular wifi signal so I blocked out the ship's wifi and was then able to use my phone both as a phone and as a wifi hotspot.

Basically, I agree with what you said except on my ship the cruise line says if my phone shows "Cellular at Sea" as the service provider, the cruise line doesn't benefit and the Cellular at Sea company bills me thru my phone company. Called my phone company and they said the Cellular at Sea charges would be $2.99 minute, 50 cents per sent text, and 5 cents per rec'd text, and $2.05 per megabyte of data. Then I got a text from Cellular at Sea reciting all above rates except data and the text said data was unavailable. But, yes, if you are using your phone as a hot spot offshore, you would be wise to make sure it is not running on anything other than a regular cell phone service. So if a regular cell service is not available, (or if roaming is an issue) then a person could use WhatsApp to make calls and do texts off the ship's wifi which can be a lot cheaper.

My experience with using wifi-based communication apps while on board (e.g., Skype), is that the ship blocks these.YMMV, but I would not count on them.

On another matter, I cannot imagine a company, cruise ship or not, that provides a service like Cellular at Sea without charging the service provider for the opportunity to make a profit off of its customers.

Well, what do you know! I was on a Silversea ship, the Silver Shadow, and Skype was not blocked. Thank goodness for that, for I needed to make a phone call that lasted over 45 minutes, and would have set me back $116.00 if I had to use my cell service. Thanks, Silversea!

That’s an important message shared in above posts. Don’t assume every cruise line has similar policies.

Some cruise lines now provide free internet. I hope others will be pressured into doing that by the competition.

Some cruise lines seem to be able to disable VOIP calling when you’re using their internet. Fortunately, not all do, and you may find using WhatsApp etc. may work very well.

If using cellular service because you’re offshore, etc. be sure you’re connected to your provider or it’s partners and not the company providing onboard cellular service, or you could be in for a rude surprise on your next bill.

And don't forget about network time setting. I've had my phone time off because the network time picked up was not the ship's time. Phones have setting for "time zone" that comes from the mobile network.
Don't want you to miss dinner!

Neat little thing we discovered this summer on Regent - even without having internet enabled, we were able to use imessage to text each other as long as we connected to the ship's network. I am not techy enough to understand why this worked as it's certainly not something they advertise, but it was great.

More on ship's wifi vs. local port carrier signal
 

If you are using the ship's wifi while out of range of land based carrier signals, you may need to turn off your wifi reception when you get into port or close to land to receive the local carrier signal--assuming, for example, that you are willing to pay, for example, Verizon's $10/day travel pass. If you don't turn off wifi, the phone may default to the ship's strong wifi signal, and if you are on a ship that charges for wifi and you haven't signed onto the wifi, you won't get anything. So this is a case of the ship's wifi "blocking" your port's cellular service, but it is blocking that is easily defeated and the reason for this blocking may be that the phone is programmed to default to wifi because in most cases wifi is cheaper.

Also, Seabourn insists that Cellular at Sea is entirely an offering of the cell phone company and not of the ship. I am a Verizon customer and I found Cellular at Sea offered when out of reach of normal phone service on a Canadian maritime cruise but not a Med cruise.