on my aqualine em104. whenever heat goes on get a minor shock if i touch the heater assy. replaced heater element assy last year.

1. Install GFCI that protects entire circuit.
2. With power removed from unit, do an ohms check between either of the element legs to ground to see if you get anything... put your meter on a high >200K ohms scale.

If you read anything, then you need to replace your element... it's failed, most likely due to water quality.

That should do it.

In addition to Pageup's reccomendations, ensure that the spa is properly grounded, and the receptacle is grounded properly as well. Even with an element and GFCI failure, if you're properly grounded, you should not feel any current simply by touching the equip or water.
Good luck!
~Swine

Yet another addition. You might want to make double sure your heater housing is grounded to the equipment control cabinet, either by directly mounting to the metal housing, or using an 8 gauge solid copper wire to bond the two together. Almost always if you are feeling current when the heater is on, the element will be bad, via either a rusty spot on the element or a crack. The element is actually INSIDE the metal tube you see when you have the element in your hand, and it's isolated from the tube itself. Most likely that insulation inside the tube has been compomised in some way.

a little off the normal track on this one.
As above i agree, ensure your circuit wiring and protection is correct and working,
however, don't rule out the fact that this problem may have nothing to do with your spa at all.
On occasion a bad electrical connection in a house switchboard or wiring can result in a raising of the earth potential above zero volts. this can occur with a fault to the insulation resistance of ANY electrical device in the house regardless of power consumption size. if insulation resistance is compomised in such (a fauly) installation, then when an appliance of large current draw is used (stove, hot plates, hot water system, spa heater, etc) this causes an even greater current flow through the bad insulation to earth. Anything that is earthed, then has a raised potential including taps, all metal elect appliances, water out of the shower, AND spa water - felt more-so in the case of a spa because you are likely standing on zero potential ground when touching spa water, where-as in the house, there is insulation offered by timber floors, florr coverings, shoes, etc which may make this problem un-noticeable.
as a result it is important that you discover the cause of the fault (whether it simply be the heater element or something in the house) and also address the issue as to why the earth potential has raised to be able to get a shock, because any faulty device will not normally result in a shock, it will trip the gfci or circuit breaker.