At best these are messy, at worst they will stop your engine. If the tank is below the engine any fuel leak will result in air being drawn into the system when the engine is stopped. This will make the engine very difficult to start.
Leaks can be from the low pressure side of the system, the drain lines or the high pressure side. Fittings used are normally sealed metal-to-metal or with sealing washers made from aluminium or copper.
Never use jointing compound or PTFE tape to seal fittings in the fuel system. Again, it is virtually impossible to stop tiny particles entering the system and blocking the minute, vital holes and valves in the pumps and injectors.
Fuel leaks need to be dealt with straight away as air or water drawn into the system will stop an engine, especially if the tank is below engine level. This banjo on a fuel return fitting is typical of those found on diesel engines and relies on copper washers for a good seal. Don’t use PTFE tape. Before tightening the top bolt check the washers for damage and replace them if necessary. Ideally they should be heated until red to anneal and soften the metal for better bedding down. A clean fuel system like this will be easier to check for leaks.
If a fitting or pipe cannot be sealed by tightening, check for corrosion of the aluminium washers, fatigue cracks in high pressure pipes or cracks in fittings. Replace suspect parts, taking care not to overtighten.
When leaving the boat for a few days or more remove the cover from the engine compartment to allow air to circulate. This will reduce condensation in the compartment as the engine cools, thus reducing the likelihood of corrosion.