Learn Finnish: Me ristimme uuden veneen Karhuksi. - We christened the new boat Bear.

Verbs of naming, such as ristiä (to christen), nimittää (to name/appoint), and kutsua (to call), require the assigned name or role to be in the translative case, which ends in -ksi. In Finnish logic, the object transitions into the new identity of the Bear.

Why is the object 'uuden veneen' in the genitive-accusative rather than the partitive?

The action of christening is a completed, one-time event that affects the entire object. Because the action is complete and the object is a whole unit, the total object rules apply, changing the nominative uusi vene into the genitive-like accusative uuden veneen.

Could I use the verb 'kutsua' (to call) here instead?

You could, but the grammar and meaning would shift slightly. Ristiä refers to the specific, completed act of christening. If you used kutsua to mean 'We call the new boat Bear', the action would be an ongoing, continuous state. That would require the partitive object: Me kutsumme uutta venettä Karhuksi.

Is it normal to capitalize 'Karhuksi' even with the case ending attached?

Yes. Because Karhu is being given as a proper noun (the specific name of the boat), it remains capitalized. The case ending -ksi is simply attached directly to the capitalized name.