Naval units are ? similar to air units ? very expensive. Thus it is very important to use these units effectively.
The most important naval unit is the transporter, because this is the only unit, which can transport ground units. But because of its weak defensive value it must be guarded properly. Battle boats are very effective for protection but their benefit is disproportionate to their costs. Therefore in order to increase the strength of your fleet the optimum is to purchase carriers. With only ¾ of the cost of a battle boat you can gain a three times higher defensive value after landing of fighters.
A rule of thumb for transporters says that you should just have enough transporters to ship all units that are produced.
But this is not the whole story. For UK and especially for US (because the US turn is after an eventual support from Japan) it might be a good idea to buy additional transporters in order to increase the pressure on GER and WEU. However additional transporters usually don?t increase the pressure on EEU.
In case your own naval units are threatened, the first thing you should look after is, if the threat will be constant (by example US-bmb in NOV, but US has to retreat within the next turn). If there is a constant threat it is recommended to purchase new fleet. On the other hand if the threat is only temporary the threat can be diminished by repositioning your own units.
If Japan sends units to Africa via BUR/SCH it is an advantage to have additional units present in Japan. In case of a retreat from Africa or the present units in Africa are sufficient the transporters usually used to transport these units to Africa can now be utilized to send additional units from Japan to Asia. In general the purchase of submarines is not advisable, although it could be of advantage for US and Japan during a pacific strategy. In the Atlantic production of submarines is only of interest, if Japan builds an Industrial complex in BRA.
Conclusion: An optimal utilization of transporters and an adequate cover of the fleet against enemies are the key points to consider.