Here's the highest i can see Hitler being in the navy.
The POD of course is that he serves in the navy blah blah blah. He survives the war and witnesses the Kiel mutiny. In the aftermath of WW1 Germany goes into its revolutionary period. Hitler reflects on his experience in the mutiny and has a great dislike of the mutineers. Rather than sitting around passively, he joins Marinebrigade Ehrhardt and takes part in the Freikorps operations in fighting German revolutionaries, along with fighting in Poland and Czechoslovakia.
Perhaps before the Kapp putsch, Hitler has a break with Ehrhardt due to the latter's very monarchist views. Hitler leaves the Marinebrigade but uses his public speaking skills to get a decent sized amount of Freikorps members to leave with him. Marinebrigade Ehrhardt takes part in the Kapp putsch as irl and ends up disbanded with Ehrhardt fleeing the country. Although Ehrhardt sets up the Organization Consul terrorist organization, the majority of remnants of ex Marinebrigade Ehrhardt join Hitler's breakaway Freikorps instead.
So yeah, basically the highest i see Hitler going in the navy is Freikorps leader, and that isnt even actual navy.
Ironically this Hitler might be a high ranking member of the SA. Actually, lets roll with that.
Hitler joins the Nazi party and brings his Freikorps along with him. Hitler becomes one of the Party's best speakers, but thanks to his Marinebrigade also takes a large roll in the matters of the armed branch of the Nazi party (ie the SA). Marinebrigade Hitler (for lack of a better name), probably turns into the SS equivalent in this timeline. Himmler will most likely not come to power since Hitler wants to run his unit personally. Due to that the SS/Marinebrigade doesnt break into a seperate organization from the SA, 'merely' being considered the elite bodyguard unit of the SA.
I think Rohm would influence Hitler a bit here, since rather than being just old friends they would also be comrades in arms due to mutual SA membership. Hitler's domestic policies might be a lot more populist, with some more socialism in the national socialist.
Eventually Rohm's radicalism is going to bite him in the ass. Hitler will probably try his best to calm his old friend down (and due to also being his superior officer might actually succeed). Several ways this could go i suppose. Hitler manages to make a powersharing agreement and the SA is merged with the army as part of rearmament. Or maybe Rohm ends up being dismissed if he refuses to work with the army and the SA basically becomes a much larger SS, and it being a backup army for the Heer. I dont see Hiter outright dismantling the SA like OTL, if only for sentimental reasons.
Effects on the war? The chances of an army coup if things go bad is probably more likely, since they have another reason to dislike him. Even if he tries to treat both the SA and army fairly the officer corp isnt going to like that. Not to mention his navy connections.
I suppose WW2 might be postponed if Hitler decides to focus more on domestic affairs thanks to the Rohm connection.
Hitler would also have an interest in 'nazifying' the Navy. The memory of the Kiel mutiny in the last war is still clear in his mind. There might be some purges of the navy. On the other hand he might have some favoritism towards the navy as well, ordering the construction of more vessels, and giving the navy an infantry and aviation arm (which might count as nazification of the navy, since those branches of the navy would probably be more national socialist).