Display duplicate row indicator and get only one row when duplicate

I built the schema at http://sqlfiddle.com/#!18/7e9e3

CREATE TABLE BoatOwners 
(
     BoatID INT,
     OwnerDOB DATETIME,
     Name VARCHAR(200)
);

INSERT INTO BoatOwners (BoatID, OwnerDOB,Name)
VALUES (1, '2021-04-06', 'Bob1'),
       (1, '2020-04-06', 'Bob2'),
       (1, '2019-04-06', 'Bob3'),
       (2, '2012-04-06', 'Tom'),
       (3, '2009-04-06', 'David'),
       (4, '2006-04-06', 'Dale1'),
       (4, '2009-04-06', 'Dale2'),
       (4, '2013-04-06', 'Dale3');

I would like to write a query that would produce the following result characteristics :

1. Returns only one owner per boat
2. When multiple owners on a single boat, return the youngest owner.
3. Display a column to indicate if a boat has multiple owners.

So the following data set when apply that query would produce

I tried

ROW_NUMBER() OVER (PARTITION BY ....

but haven’t had much luck so far.

Answer

This is just a case of numbering the rows for each BoatId group and also counting the rows in each group, then filtering accordingly:

select BoatId, OwnerDob, Name, Iif(qty=1,'No','Yes') MultipleOwner 
from (
    select *, Row_Number() over(partition by boatid order by OwnerDOB desc)rn, Count(*) over(partition by boatid) qty
    from BoatOwners
)b where rn=1