Your Naval Doctrine influences your naval combat, it determines what kind of fleet you'll want to build and how you'll want to use it.
Naval Doctrine effects:
- Organisation and Morale values.
- Convoy Raiding and Convoy Escort efficiency.
- ASW efficiency.
- CV strike efficiency.
- Positioning. This value is critically important, check out the Naval Primer for further explanation.
There are 3 Naval Doctrine trees available, and they are in no way equal.
- Germany practices Sealane Interdiction (SLI).
- Britain, France, Italy, Australia, Canada, Netherlands, South Africa and the SU starts with Fleet In Being (FIB).
- The US and Japan go for Base Strike (BS...).
While each tech tree excels in a certain area, it is quite clear that BS > FIB > SLI whatever you want to do. Gaining naval superiority is always better than working around it.
SLI is the least popular doctrine, with Germany as its sole worshiper. It emphasises convoy raiding with U-boats, basically weakening the naval might of an enemy you're never going to surpass. It is the shortest tree out of the three, ending in 1942 and gaining most of its bonuses by 1940, when it even outshines the other two doctrines for a brief period. SLI offers the best morale, the best SS positioning, and the best convoy raiding efficiency.
A quick note about raiding and U-boats: Raiding won't make Britain run out of resources because of the pre-war buildup. It's more than capable of stopping supply convoys though, thereby hampering their ability to wage land wars overseas. DD seriously crippled the U-boat's sea attack rating, introduced sonar and ASW upgrades for ships, and added the U-boatkiller CVL into the mix. U-boats are still the best option for convoy raiding, but they will not destroy the enemy fleet like in HoI2. You should abandon this tree if you're going for naval supremacy.
FIB focuses on naval supremacy through (non CV) capital ships, and helps to maintain your colonial empire. It has the best BB and BC positioning scores, the best organisation, and the best convoy escort and ASW ratings. It will also gain convoy the escort bonus slightly earlier than BS, and it's a nice counter to SLI. This tree ends in 1944.
Base Strike is the longest and most powerful of the three tech trees. It focuses on winning naval battles with the help of Carriers. It offers the best CV and DD positioning by far, and the best CV strike efficiency. It also has a convoy raiding bonus (although weaker than SLI), and the convoy escort + ASW bonus (although slightly weaker than FIB). The tree ends in 1947 (or 1952 of you consider the semi unrelated amphibious bonus). It will always have the best CV and DD positioning bonus. CVs are the key to naval victory, and therefore this doctrine tree is the key to naval supremacy.
Note that even though Carriers receive snow, rain and misc bonuses, this will not make them effective in bad weather. They are still sitting ducks when it's snowing.
Abandoning Your Doctrine
It is usually best for you to stay where you are. But if you decide switch, you should obviously switch to Base Strike.
The early bonus of SLI allows you to attempt an early Seelöve at the start of the war, if you start building ships early. This is your only chance though, and it's fate will most likely decide your multiplayer match. Or you could focus on land as you should, and raid Allied commerce with U-boats (or even ignore your navy altogether). Your tech teams are well suited to research SLI, but not BS. You will be dependant on Japanese blueprints to get far in the BS tree, and you'll have to neglect some of your precious air/land research. I wouldn't bother abandoning, as it's the Soviet invasion that decides most games anyway. You will have to give up on early naval operations, and four techs you start with to go for Base Strike...
Your tech teams are disastrous, but their expertises are somewhat useful in FIB. Not quite so in BS. You start with a sizeable non CV fleet, and FIB goes well with that. Chances are your sea battle will be over long before you could go anywhere deep into BS or ally with Japan for blueprints. Abandoning your two starting techs will probably hurt more than you could gain in the long run.
You start with an awesome fleet of battleships, and you're not likely to lose a naval battle against Germany or Italy anyway. The ASW and escorting bonus could help a lot too. The UK gets a nice BS tech team in 1943, and can hope to get blueprints from the US as well. You should consider switching if you're planning to move against the US later in the game, or maybe if a super Japan emerges through some weird twist of fate.
Doctrine Trees in Depth
Fleet In Being