I tend to agree with you however it seems a bit disingenuous not to mention the foreign excursions. I would feel the same about any other charity which is nominally a national charity.It's a fair point, however in counter to it I would suggest that a huge number of charities have multi-national operations and it shouldn't come as too much of a surprise that the RNLI does anyway.
As the UK we are highly regarded in many areas; I don't think that it is such a great revelation that a very-well established institution such as the RNLI share their expertise abroad.
If the RNLI were to cover the costs which their overseas expedition incur would that be unreasonable? Particularly if they're losing money here. That would be a thought.
maybe just tell people to stay away from the water rather than encourage people to swim in it. That's just asking for trouble