Why is water profusely coming out of my shower head when the water is boiling in my water heater?
I just got my water heater fixed as previously it wasn't boiling water


A water heater should not raise the temperature of the water to 100℃ (boiling). You should not have any boiling occurring anywhere inside your household hot-water systems (excluding vessels used for preparation of food or hot drinks such as kettles and saucepans). 

In my part of the world, a home water heater has a thermostat that determines the maximum temperature of the water, this thermostat cuts off the source of heat when the water reaches the set temperature. Perhaps you have a faulty thermostat, the wrong type of thermostat or one set to an incorrect value?


  is it safe for my family to take a shower while its like this?


There are two main health aspects of water heating systems:


preventing pathogens multiplying in the water
preventing people from being scalded


The first, primarily concerning Legionalla, is prevented by storing water at ⋝60℃ and ensuring it is ⋝55℃ at outlets.

The latter conflicts somewhat with the first. You need lower temperatures to prevent scalding of vulnerable individuals (babies, elderly, etc) The UK HSE suggests water at the outlet needs to be less than 44℃ to prevent scalding.

One way to reconcile this may be to have a thermostatic mixer close to the outlet - this will mix a small mount of cold water with the hot to reduce the temperature slightly. One place where you normally find these sorts of thermostatic mixer in the UK is under washbasins inside washrooms for the disabled. In most houses though, the temperature is just a compromise between these two risks and is not regulated using a thermostatic mixer. 

Many showers include a thermostatic control that prevents the shower producing water hot enough to cause scalding.


  I'm guessing theres something wrong with the pressure seal on the shower head?


As Mike Baranczak commented, this is almost certainly not the cause of your problem.

In the types of shower familiar to me, the shower head plays no role in shutting off the supply of water. You might have a shower head that can be rotated to produce various patterns of spray, some feeling softer and some feeling more vigourous, but this doesn't much affect the flow rate or shut off the flow.

You shut off the flow of water using a valve much lower down than the shower head. This valve is part of the shower controls that you use to set temperature and flow rate. There is a valve that is either directly operated or operated by solenoid (often the case in electric showers which use electricity to heat an insulated element that heats cold water passing through the shower unit)


  Might it be that the pressure in the water tank is forcing out the hot water as the water begins to get very hot?


The water heater should have some means of handling excess pressure. Often it will have a pressure relief valve that vents excess pressure safely. Some older designs have the pressure regulated by a header tank.

If the pressure is high enough to force excessively hot water past a shut valve, there is probably something wrong with the heater, the valve or both.

If you need more specific help, you could edit your question to add details of make and model of heater and shower contols and shower head, together with photos of those items.