Commercial model submarine hulls are nearly always made from fibreglass or GRP (Glass Fibre Reinforced Plastic). This is well suited for model submarine hulls and It can easily be formed using male and female moulds. Once cured it resists attack from most chemicals. It can be sawn, drilled, filed, filled, glued and painted. If damaged it can be quickly and easily repaired. It also handles the extremes of temperature very well. Its other major properties include a very high strength to weight ratio and almost zero water absorption when fully covered by the gel coat.
With an all fibreglass hull, the bonding of the parts simply require more fibreglass resin or some form of epoxy adhesive such as Araldite. If you use Araldite be sure to get the 24-hour version and not the 1-hour, fast or rapid set types as these are usually not always fully waterproof. Another material that should be avoided in areas where strength and waterproofing is paramount is the P38 type of car body fillers. This material is only filler has no strength and even when painted it can absorb water. So far the best material used for fillers are the epoxy putties such as Milliput. This looks like two sticks of coloured plasticine; they are mixed together to a uniform colour and then applied like putty. Then it can easily be smoothed out and shaped with either modelling tools or a wet finger, once set it can be drilled, filled and sanded just like fibreglass, wood or metal.
The boat in this picture was made from balsa wood, sealed with aircraft dope and tissue and then painted. It was built from a free plan named 'HMS Unseen', which appeared in a Submarine Special edition of Marine Modelling International magazine.
If you are interest in building this, you can purchase the plan from HERE