I started playing when races got race-specific characters and things that nobody else got; those units help provide character to the race. Now it's all "counts-as" and nothing is sacred
Not all SM armies get to use the Blood Angel fast vehicles or Dark Angel Mortis dreadnoughts, despite their superior qualities. It seems more fluffy that way, in that communication isn't instant-galaxy-wide and things stay hidden or jealously guarded by their chapters. Similarly, not all SM chapters will have such a large gunboat, mainly because it is very unfluffy. The IN and Inquisition and whoever else wouldn't allow it, not to mention not all forgeworlds are the same and can produce that kind of thing.
What's so wrong with painting it in Ultramarines colors and saying it's joined up with your chapter for a crusade or somesuch? If anything it gives you the opportunity to try a different paint scheme. Trading it out for a regular battlebarge is only a difference of 25 points, which can go toward a reroll.
In the end it's your fleet, and as such it's not subject to moral complaints by anyone but yourself